We want it to be possible to bulid with clang, so also check against that.

# Pascal Jäger <pascal.jaeger@leimstift.de>

--- a/configure.ac
+++ b/configure.ac
@@ -303,7 +303,7 @@ done
 
 AC_DEFUN([AC_CHECK_GCC_ARG],
  [if test -n "$GCC"; then
-   AC_CACHE_CHECK([whether gcc accepts [$2]],
+   AC_CACHE_CHECK([whether $[$CC] accepts [$2]],
      ac_cv_gcc_accepts_[$1],
     [rm -rf conftest.$ac_ext
      touch conftest.$ac_ext
@@ -316,7 +316,7 @@ AC_DEFUN([AC_CHECK_GCC_ARG],
      # earlier diagnostics".
      #
      without_no=`echo $2 | sed s/Wno-/W/`
-     if ( ( gcc -c $without_no conftest.$ac_ext -o/dev/null >/dev/null ) \
+     if ( ( $CC -c $without_no conftest.$ac_ext -o/dev/null >/dev/null ) \
           2>&1 | \
           grep unrecognized >/dev/null ); then
        ac_cv_gcc_accepts_[$1]=no