Save config.cache before tests which are likely to fail.

Guile's configure script takes fairly long.  On a system without
automatic build dependency installation, it is amplified by the fact
that your user might be both impatient and not read the build
instructions, thus needs to rerun configure several times only to find
out there is another dependency to install.  Still, there is help even
for such kinds of misguided figures such as me: save the temporary
config.cache file (if one is used anyway) before embarking upon the set
of tests that are likely to error out.  That way, the second and further
reruns are much faster.

The downside of this patch is that, when fixing the errors requires
changing to precious variables (such as CPPFLAGS or so), the user will
need to 'rm -f config.cache' to keep configure happy.

* configure.ac: Call AC_CACHE_SAVE before the gmp tests.

configure.ac

@@ -855,6 +855,7 @@ main (void)
   esac
 fi
 
+AC_CACHE_SAVE
 
 dnl GMP tests
 AC_LIB_HAVE_LINKFLAGS([gmp],