AddressSanitizer's (ASAN) SIGSEGV handler overrides the backtrace
handler and prevents it from printing its backtrace. The output of ASAN
is different from what 'bt_test.py' expects and causes backtrace test
failures.
The 'allow_user_segv_handler' option allows applications to set their
own SIGSEGV handler but is not supported by older GCC versions. These
older GCC versions do support the 'handle_segv' which prevents ASAN from
setting its SIGSEGV handler.
Use environment variables instead. This repairs 'make distcheck',
which was running into trouble when it tried to chmod the generated
scripts.
Fixes 17148.
