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: another implementation was submitted, modified by
maintainer Roy, then simplified based on my suggestion. This patch is a fold
of the two upstream patches:
http://roy.marples.name/projects/dhcpcd/info/5f4bf91142fceac502c478886aeb5079a184bb43
http://roy.marples.name/projects/dhcpcd/info/26bc10be2405659bdf31d690a72d377d6b67ebd4

diff -r 66c8bdc88858 configure
--- a/configure	Sat Jun 21 16:54:30 2014 +0200
+++ b/configure	Sun Jun 22 15:20:25 2014 +0200
@@ -272,8 +272,9 @@
 fi
 
 echo "Using compiler .. $CC"
-if ! type "$CC" >/dev/null 2>&1; then
-	echo "$CC is not an executable"
+if ! echo "int main(void) { return 0; }" | $CC -x c -o /dev/null - 2>/dev/null
+then
+	echo "$CC does not create executables"
 	exit 1
 fi
 [ "$CC" != cc ] && echo "CC=	$CC" >>$CONFIG_MK