build system: use pkg-config to look up selinux libs

Newer versions of libselinux has started linking against more libs.
Rather than continuing hardcoding things, switch to using pkg-config
to query for its dependencies.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/Makefile b/Makefile
index b2e9477..8995ff5 100644
--- a/Makefile
+++ b/Makefile
@@ -297,6 +297,7 @@
 STRIP		= $(CROSS_COMPILE)strip
 OBJCOPY		= $(CROSS_COMPILE)objcopy
 OBJDUMP		= $(CROSS_COMPILE)objdump
+PKG_CONFIG	?= $(CROSS_COMPILE)pkg-config
 AWK		= awk
 GENKSYMS	= scripts/genksyms/genksyms
 DEPMOD		= /sbin/depmod