Use cross-compilation-safe pkg-config macro.

origin commit from:
dropbox/nautilus-dropbox#63

configure.ac

@@ -21,11 +21,7 @@ AC_PROG_CC
 AM_PROG_LIBTOOL
 
 # Check for pkg-config
-AC_CHECK_PROG(HAVE_PKGCONFIG, pkg-config, yes, no)
-AC_PATH_PROG(PKG_CONFIG, pkg-config, no)
-if test "x$HAVE_PKGCONFIG" = "xno"; then
-	AC_MSG_ERROR(you need to have pkgconfig installed !)
-fi
+PKG_PROG_PKG_CONFIG
 
 PKG_CHECK_MODULES(CAJA, libcaja-extension >= $CAJA_REQUIRED)
 PKG_CHECK_MODULES(GLIB, glib-2.0 >= $GLIB_REQUIRED)