diff --git a/changes/bug19139 b/changes/bug19139
new file mode 100644
index 0000000000..2e53d7d445
--- /dev/null
+++ b/changes/bug19139
@@ -0,0 +1,3 @@
+  o Minor features (build):
+    - Use the Autoconf macro AC_USE_SYSTEM_EXTENSIONS to automatically
+      turn on C and POSIX extensions. Closes ticket 19139.
diff --git a/configure.ac b/configure.ac
index 386fcdd132..6d721a2fc5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -16,6 +16,7 @@ AM_INIT_AUTOMAKE([foreign 1.11 subdir-objects])
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
 AC_CONFIG_HEADERS([orconfig.h])
 
+AC_USE_SYSTEM_EXTENSIONS
 AC_CANONICAL_HOST
 
 PKG_PROG_PKG_CONFIG
@@ -1552,9 +1553,6 @@ int main(int c, char **v) { puts(__FUNCTION__); }])],
 AC_CACHE_CHECK([whether we have extern char **environ already declared],
   tor_cv_have_environ_declared,
   AC_COMPILE_IFELSE([AC_LANG_SOURCE([
-/* We define _GNU_SOURCE here because it is also defined in compat.c.
- * Without it environ doesn't get declared. */
-#define _GNU_SOURCE
 #ifdef HAVE_UNISTD_H
 #include <unistd.h>
 #endif
diff --git a/m4/pc_from_ucontext.m4 b/m4/pc_from_ucontext.m4
index 6bedcbb2da..8a9dc459e6 100644
--- a/m4/pc_from_ucontext.m4
+++ b/m4/pc_from_ucontext.m4
@@ -79,24 +79,21 @@ AC_DEFUN([AC_PC_FROM_UCONTEXT],
      if ! $pc_field_found; then
        # Prefer sys/ucontext.h to ucontext.h, for OS X's sake.
        if test "x$ac_cv_header_cygwin_signal_h" = xyes; then
-         AC_TRY_COMPILE([#define _GNU_SOURCE 1
-                         #include <cygwin/signal.h>],
+         AC_TRY_COMPILE([#include <cygwin/signal.h>],
                         [ucontext_t u; return u.$pc_field == 0;],
                         AC_DEFINE_UNQUOTED(PC_FROM_UCONTEXT, $pc_field,
                                            How to access the PC from a struct ucontext)
                         AC_MSG_RESULT([$pc_field])
                         pc_field_found=true)
        elif test "x$ac_cv_header_sys_ucontext_h" = xyes; then
-         AC_TRY_COMPILE([#define _GNU_SOURCE 1
-                         #include <sys/ucontext.h>],
+         AC_TRY_COMPILE([#include <sys/ucontext.h>],
                         [ucontext_t u; return u.$pc_field == 0;],
                         AC_DEFINE_UNQUOTED(PC_FROM_UCONTEXT, $pc_field,
                                            How to access the PC from a struct ucontext)
                         AC_MSG_RESULT([$pc_field])
                         pc_field_found=true)
        elif test "x$ac_cv_header_ucontext_h" = xyes; then
-         AC_TRY_COMPILE([#define _GNU_SOURCE 1
-                         #include <ucontext.h>],
+         AC_TRY_COMPILE([#include <ucontext.h>],
                         [ucontext_t u; return u.$pc_field == 0;],
                         AC_DEFINE_UNQUOTED(PC_FROM_UCONTEXT, $pc_field,
                                            How to access the PC from a struct ucontext)
diff --git a/src/common/backtrace.c b/src/common/backtrace.c
index 3b762b68e3..afdc10d6a9 100644
--- a/src/common/backtrace.c
+++ b/src/common/backtrace.c
@@ -13,9 +13,6 @@
  * detect crashes.
  */
 
-#define __USE_GNU
-#define _GNU_SOURCE 1
-
 #include "orconfig.h"
 #include "compat.h"
 #include "util.h"
diff --git a/src/common/compat.c b/src/common/compat.c
index 370881b07e..72dffe2a6e 100644
--- a/src/common/compat.c
+++ b/src/common/compat.c
@@ -12,18 +12,6 @@
  * the platform.
  **/
 
-/* This is required on rh7 to make strptime not complain.
- * We also need it to make memmem get defined (where available)
- */
-
-/* XXXX We should just  use AC_USE_SYSTEM_EXTENSIONS in our autoconf,
- * and get this (and other important stuff!) automatically. Once we do that,
- * make sure to also change the extern char **environ detection in
- * configure.ac, because whether that is declared or not depends on whether
- * we have _GNU_SOURCE defined! Maybe that means that once we take this out,
- * we can also take out the configure check. */
-#define _GNU_SOURCE
-
 #define COMPAT_PRIVATE
 #include "compat.h"
 
diff --git a/src/common/compat_pthreads.c b/src/common/compat_pthreads.c
index 1b24cc3c2a..1d6db34225 100644
--- a/src/common/compat_pthreads.c
+++ b/src/common/compat_pthreads.c
@@ -10,8 +10,6 @@
  * functions.
  */
 
-#define _GNU_SOURCE
-
 #include "orconfig.h"
 #include <pthread.h>
 #include <signal.h>
diff --git a/src/common/compat_threads.c b/src/common/compat_threads.c
index 8f9001258a..9c6fa55b32 100644
--- a/src/common/compat_threads.c
+++ b/src/common/compat_threads.c
@@ -11,8 +11,6 @@
  *  modules.)
  */
 
-#define _GNU_SOURCE
-
 #include "orconfig.h"
 #include <stdlib.h>
 #include "compat.h"
diff --git a/src/common/crypto.c b/src/common/crypto.c
index f47f5d2471..d5043c3246 100644
--- a/src/common/crypto.c
+++ b/src/common/crypto.c
@@ -56,7 +56,6 @@ ENABLE_GCC_WARNING(redundant-decls)
 #include <ctype.h>
 #endif
 #ifdef HAVE_UNISTD_H
-#define _GNU_SOURCE
 #include <unistd.h>
 #endif
 #ifdef HAVE_FCNTL_H
diff --git a/src/common/sandbox.c b/src/common/sandbox.c
index 4e2c5cde22..94b2fc6788 100644
--- a/src/common/sandbox.c
+++ b/src/common/sandbox.c
@@ -39,8 +39,6 @@
 
 #if defined(USE_LIBSECCOMP)
 
-#define _GNU_SOURCE
-
 #include <sys/mman.h>
 #include <sys/syscall.h>
 #include <sys/types.h>
diff --git a/src/common/sandbox.h b/src/common/sandbox.h
index 2defd8bbd4..c5963e3119 100644
--- a/src/common/sandbox.h
+++ b/src/common/sandbox.h
@@ -39,12 +39,6 @@ typedef struct sandbox_cfg_elem sandbox_cfg_t;
  */
 #ifdef USE_LIBSECCOMP
 
-#ifndef __USE_GNU
-#define __USE_GNU
-#endif
-#ifndef _GNU_SOURCE
-#define _GNU_SOURCE
-#endif
 #include <sys/ucontext.h>
 #include <seccomp.h>
 #include <netdb.h>
diff --git a/src/common/util.c b/src/common/util.c
index 2f7cc4f892..7217cdc81e 100644
--- a/src/common/util.c
+++ b/src/common/util.c
@@ -9,10 +9,6 @@
  * process control.
  **/
 
-/* This is required on rh7 to make strptime not complain.
- */
-#define _GNU_SOURCE
-
 #include "orconfig.h"
 #ifdef HAVE_FCNTL_H
 #include <fcntl.h>
diff --git a/src/ext/eventdns.c b/src/ext/eventdns.c
index 7046f32eb9..f5b7723b54 100644
--- a/src/ext/eventdns.c
+++ b/src/ext/eventdns.c
@@ -50,9 +50,6 @@
 #endif
 #endif
 
-/* #define _POSIX_C_SOURCE 200507 */
-#define _GNU_SOURCE
-
 #ifdef DNS_USE_CPU_CLOCK_FOR_ID
 #ifdef DNS_USE_OPENSSL_FOR_ID
 #error Multiple id options selected