Tweak and cleanup AC_FIND_GAP macro

The only user visible change is that now instead of
  "GAP was compiled without GMP"
we print the less ambiguous message
  "GAP was compiled without its own GMP"

m4/ac_find_gap.m4

@@ -1,6 +1,6 @@
 # Find the location of GAP
 # Sets GAPROOT, GAPARCH and GAP_CPPFLAGS appropriately
-# Can be configured using --with-gaproot=... and --with-configname=...
+# Can be configured using --with-gaproot=... and CONFIGNAME=...
 #######################################################################
 
 AC_DEFUN([AC_FIND_GAP],
@@ -152,7 +152,7 @@ AC_DEFUN([AC_FIND_GAP],
     GAP_CPPFLAGS="$GAP_CPPFLAGS -I$ARCHPATH/extern/gmp/include"
     AC_MSG_RESULT([$ARCHPATH/extern/gmp/include/gmp.h])
   else
-    AC_MSG_RESULT([not found, GAP was compiled without GMP])
+    AC_MSG_RESULT([not found, GAP was compiled without its own GMP])
   fi;
 
   AC_SUBST(GAPARCH)