GCC: add -std=gnu99 to CFLAGS

- this change prevents issues like #1539 in the future
- Clang's default is already gnu99 but GCC's is gnu89
- mruby is C99 (+ GNU exts), not C89 (+ GNU exts)
Merge pull request #1586 from cremno/gcc-std-gnu99

GCC: add -std=gnu99 to CFLAGS