selftest/mqueue: enable cross compilation

Use the CC variable instead of hard coding gcc. Also clean up the compiler
options by creating a CFLAGS variable.
Signed-off-by: NTyler Baker <tyler.baker@linaro.org>
Signed-off-by: NShuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/mqueue/Makefile

+CFLAGS = -O2
+
 all:
-	gcc -O2 mq_open_tests.c -o mq_open_tests -lrt
-	gcc -O2 -o mq_perf_tests mq_perf_tests.c -lrt -lpthread -lpopt
+	$(CC) $(CFLAGS) mq_open_tests.c -o mq_open_tests -lrt
+	$(CC) $(CFLAGS) -o mq_perf_tests mq_perf_tests.c -lrt -lpthread -lpopt
 
 include ../lib.mk