code: def build_extensions (self):
          
          # For gcc >= 3 we can add -std=gnu99 to get rid of warnings.
          extra = []
          if self.compiler.compiler_type == 'unix':
              option = "-std=gnu99"
              if cc_supports_option(self.compiler.compiler, option):
                  extra.append(option)
          # First, sanity-check the 'extensions' list
          self.check_extensions_list(self.extensions)
          for ext in self.extensions:
              for opt in extra:
                  if opt not in ext.extra_compile_args:
                      ext.extra_compile_args.append(opt)
              self.build_extension(ext)
docs: Add -std=gnu99 to build options if supported.