Use -std=gnu99 instead of -std=c99

[nurse]

If you use -std=c99, GCC doesn't use optimizations which use compiler builtin functions instead of C library.

https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html https://bugs.ruby-lang.org/issues/12336

[soutaro]

@nurse Seems like it's still failing? https://github.com/ruby/ruby/runs/5869073187?check_suite_focus=true#step:16:301

[nobu]

The reason is test-bundled-gems.rb runs the built ruby without fake.rb to build the gems. Now these extensions are generated under .bundle directory by make build-exts, and the test can use them. I’m working on it.