ensure __GLIBC__ properly defined before using it

[remicollet]

Discovered while working a a build warning

/dev/shm/BUILD/php-pecl-datadog-trace-0.15.1/NTS/src/ext/backtrace.c:54:6: warning: type of 'TSRMLS_D' defaults to 'int' [-Wimplicit-int]
 void ddtrace_install_backtrace_handler(TSRMLS_D) {
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Tracking this issue, the bas code is selected as GLIBC is tested before all include, where it is defined.

[SammyK]

Thanks so much for the PR @remicollet! I wan't able to get it to compile in the containers until I moved the #include <execinfo.h> line inside the ifdef. If you can make that tweak, I'll merge. Thanks again for the contribution! :)

[remicollet]

It seems github have very strange behavior... BTW, requested change is done

[SammyK]

Thanks @remicollet! :) Yeah, apparently there's some issues with GitHub at the moment.