configure: Test GCC-specific compiler flags only when using GCC

Commit 23373d01d "configure: Test compiler flags" (v4.4.0) made
configure test whether compiler flags work, to let us use flags that
work only for some compilers.

Unfortunately, the Sun C compiler accepts invalid options with a
warning, breaking the test.  Observed on Solaris 10.

Fix by testing the compiler flags only when AC_PROG_CC detected GCC.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>

configure.ac

@@ -91,6 +91,9 @@ MY_WITH_READLINE
 
 ### Checks for typedefs, structures, and compiler characteristics
 
+if test "$GCC"
+then
+
 # Enable useful warnings
 # Some of them are commented out because the code needs cleanup first
 # Clang needs to be tested with -Werror=unknown-warning-option
@@ -168,6 +171,8 @@ main(int argc, char *argv[])
 }
     ]])])
 
+fi # $GCC
+
 
 ### Checks for library functions