compat: don't include compat.h from the feature test source

The feature test code should test the build environment, and none of
the compat code should interfere with that. Don't include compat.h
from the feature test source. There should be no functional changes
here, but this is just the right thing to do.

compat/have_timegm.c

@@ -1,5 +1,4 @@
 #include <time.h>
-#include "compat.h"
 
 int main()
 {