tools/thermal: tmon: allow $(CC) to be defined externally

It can be helpful, especially when using a build system, to set the C
compiler externally.
Signed-off-by: NMarkus Mayer <mmayer@broadcom.com>
Signed-off-by: NZhang Rui <rui.zhang@intel.com>

tools/thermal/tmon/Makefile

@@ -8,7 +8,7 @@ WARNFLAGS=-Wall -Wshadow -W -Wformat -Wimplicit-function-declaration -Wimplicit-
 CFLAGS+= -O1 ${WARNFLAGS}
 # Add "-fstack-protector" only if toolchain supports it.
 CFLAGS+= $(call cc-option,-fstack-protector)
-CC=$(CROSS_COMPILE)gcc
+CC?= $(CROSS_COMPILE)gcc
 
 CFLAGS+=-D VERSION=\"$(VERSION)\"
 LDFLAGS+=