configure: support builds using ccache

The configure script checks whether CC is executable by means of a check
'type $CC'. However, when using ccache, CC is normally set to "ccache gcc"
(using the appropriate paths). Such a compound string is not recognized by
type, however, and thus configure bails out.

This patch changes the check 'type $CC' with a compile test.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>

Upstream status: backport of
http://roy.marples.name/projects/dhcpcd/info/5f4bf91142fceac502c478886aeb5079a184bb43

Note: a simplification of this patch was accepted upstream, but turns out to be
non-usable by the Xtensa architecture. This was also reported upstream and
should normally be reverted soon.

diff -r 66c8bdc88858 configure
--- a/configure	Sat Jun 21 16:54:30 2014 +0200
+++ b/configure	Sun Jun 29 15:42:08 2014 +0200
@@ -272,8 +272,20 @@
 fi
 
 echo "Using compiler .. $CC"
-if ! type "$CC" >/dev/null 2>&1; then
-	echo "$CC is not an executable"
+cat <<EOF >_test.c
+int main(void) {
+	return 0;
+}
+EOF
+_CC=true
+if $CC _test.c -o _test >/dev/null 2>&1; then
+	[ -x _test ] || _CC=false
+else
+	_CC=false
+fi
+rm -f _test.c _test
+if ! $_CC; then
+	echo "$CC does not create executables"
 	exit 1
 fi
 [ "$CC" != cc ] && echo "CC=	$CC" >>$CONFIG_MK