When cross compiling on mingw32, make sure it finds a CC

Makefile

@@ -534,6 +534,10 @@ ifeq ($(PLATFORM),mingw32)
     endif
   endif
 
+  ifeq ($(CC),)
+    $(error Cannot find a suitable cross compiler for $(PLATFORM))
+  endif
+
   BASE_CFLAGS = -Wall -fno-strict-aliasing -Wimplicit -Wstrict-prototypes \
     -DUSE_ICON