Remove duplicate define of __GNUC_PREREQ

Back in this commit:

  commit b436a8ae
  Author: Fabian Freyer <fabian.freyer@physik.tu-berlin.de>
  Date:   Thu Jun 9 00:50:35 2016 +0000

    gnulib: add getopt module

config-post.h was modified to define __GNUC_PREREQ, but the
original definition was never removed from internal.h, and
that is now dead code since config.h is always the first file
included.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

config-post.h

@@ -75,11 +75,11 @@
 #endif /* LIBVIRT_NSS */
 
 /*
- * Define __GNUC__ to a sane default if it isn't yet defined.
+ * Define __GNUC_PREREQ to a sane default if it isn't yet defined.
  * This is done here so that it's included as early as possible; gnulib relies
  * on this to be defined in features.h, which should be included from ctype.h.
  * This doesn't happen on many non-glibc systems.
- * When __GNUC__ is not defined, gnulib defines it to 0, which breaks things.
+ * When __GNUC_PREREQ is not defined, gnulib defines it to 0, which breaks things.
  */
 #ifdef __GNUC__
 # ifndef __GNUC_PREREQ

src/internal.h

@@ -108,16 +108,6 @@
 
 # ifdef __GNUC__
 
-#  ifndef __GNUC_PREREQ
-#   if defined __GNUC__ && defined __GNUC_MINOR__
-#    define __GNUC_PREREQ(maj, min)                                        \
-    ((__GNUC__ << 16) + __GNUC_MINOR__ >= ((maj) << 16) + (min))
-#   else
-#    define __GNUC_PREREQ(maj, min) 0
-#   endif
-
-#  endif /* __GNUC__ */
-
 /**
  * ATTRIBUTE_UNUSED:
  *