Compile in C99 mode

Author: Michael Kipper

ext/semian/extconf.rb

@@ -23,7 +23,7 @@
 have_func 'rb_thread_blocking_region'
 have_func 'rb_thread_call_without_gvl'
 
-$CFLAGS = "-D_GNU_SOURCE -Werror -Wall "
+$CFLAGS = "-D_GNU_SOURCE -Werror -Wall -std=gnu99 "
 if ENV.key?('DEBUG')
   $CFLAGS << "-O0 -g -DDEBUG"
 else