Use an up-to-date make binary in the upgrade_checks target if

one is already available.  This avoids sometimes unnecessary
step of attempting to rebuild the make binary again which may
fail at all if, for example, one has removed his /usr/include
before doing an installworld (to keep /usr/include tidied up).

Pointed out by:	kris, marcel

Makefile

@@ -141,7 +141,7 @@ kernel: buildkernel installkernel
 #
 upgrade_checks:
 	@if ! (cd ${.CURDIR}/tools/regression/usr.bin/make && \
-	    PATH=${PATH} ${MAKE} >/dev/null 2>&1); \
+	    PATH=${PATH} ${BINMAKE} >/dev/null 2>&1); \
 	then \
 	    (cd ${.CURDIR} && make make); \
 	fi