configure: respect $CC environment var override

Enables e.g. cross-compiling by setting $CC env var.  This patch
extracted from the Ubuntu package (thanks, Wookey and Colin Watson).

BugLink: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=670660
BugLink: https://bugs.launchpad.net/bugs/870197Signed-off-by: Kamal Mostafa <kamal@debian.org>

configure

@@ -3,6 +3,7 @@
 #
 INCLUDE=${1:-"$PWD/include"}
 : ${PKG_CONFIG:=pkg-config}
+: ${CC=gcc}
 echo "PKG_CONFIG:=${PKG_CONFIG}" >>Config
 
 # Make a temp directory in build tree.
@@ -19,7 +20,7 @@ int main(int argc, char **argv) {
 	return 0;
 }
 EOF
-gcc -I$INCLUDE -o $TMPDIR/atmtest $TMPDIR/atmtest.c -latm >/dev/null 2>&1 
+$CC -I$INCLUDE -o $TMPDIR/atmtest $TMPDIR/atmtest.c -latm >/dev/null 2>&1
 if [ $? -eq 0 ]
 then
     echo "TC_CONFIG_ATM:=y" >>Config
@@ -53,7 +54,7 @@ int main(int argc, char **argv)
 
 EOF
 
-if gcc -I$INCLUDE $IPTC -o $TMPDIR/ipttest $TMPDIR/ipttest.c $IPTL $(${PKG_CONFIG} xtables --cflags --libs) -ldl >/dev/null 2>&1
+if $CC -I$INCLUDE $IPTC -o $TMPDIR/ipttest $TMPDIR/ipttest.c $IPTL $(${PKG_CONFIG} xtables --cflags --libs) -ldl >/dev/null 2>&1
 then
 	echo "TC_CONFIG_XT:=y" >>Config
 	echo "using xtables"
@@ -90,7 +91,7 @@ int main(int argc, char **argv) {
 }
 
 EOF
-gcc -I$INCLUDE $IPTC -o $TMPDIR/ipttest $TMPDIR/ipttest.c $IPTL -ldl >/dev/null 2>&1
+$CC -I$INCLUDE $IPTC -o $TMPDIR/ipttest $TMPDIR/ipttest.c $IPTL -ldl >/dev/null 2>&1
 
 if [ $? -eq 0 ]
 then
@@ -130,7 +131,7 @@ int main(int argc, char **argv) {
 }
 
 EOF
-gcc -I$INCLUDE $IPTC -o $TMPDIR/ipttest $TMPDIR/ipttest.c $IPTL -ldl >/dev/null 2>&1
+$CC -I$INCLUDE $IPTC -o $TMPDIR/ipttest $TMPDIR/ipttest.c $IPTL -ldl >/dev/null 2>&1
 
 if [ $? -eq 0 ]
 then
@@ -180,7 +181,7 @@ int main(int argc, char **argv)
 	return 0;
 }
 EOF
-gcc -I$INCLUDE -o $TMPDIR/setnstest $TMPDIR/setnstest.c >/dev/null 2>&1
+$CC -I$INCLUDE -o $TMPDIR/setnstest $TMPDIR/setnstest.c >/dev/null 2>&1
 if [ $? -eq 0 ]
 then
 	echo "IP_CONFIG_SETNS:=y" >>Config
@@ -212,7 +213,7 @@ int main(void)
 #endif
 EOF
 
-if gcc -I$INCLUDE -o $TMPDIR/ipsettest $TMPDIR/ipsettest.c >/dev/null 2>&1
+if $CC -I$INCLUDE -o $TMPDIR/ipsettest $TMPDIR/ipsettest.c >/dev/null 2>&1
 then
 	echo "TC_CONFIG_IPSET:=y" >>Config
 	echo "yes"