Added -fcommon to compile option in order to make multiple definition warnings instead of errors

Since GCC 10, the new default is -fno-common, so it breaks compilation because of now unsupported "ld: multiple definition". This MR makes it compile again by forcing -fcommon to the compile chain.

(reference: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85678)

Makefile

@@ -41,7 +41,7 @@ OTHERLIBS= -lm
 
 CHIPMUNKCC = gcc
 
-CC = $(CHIPMUNKCC)
+CC = $(CHIPMUNKCC) -fcommon
 
 CHIPMUNKFLAGS = $(ARCH_OPT)