mingw build: cstool fails to build with mingw (#941)

The correct compiler was not being passed to cstool/Makefile. The expected name
for the capstone lib was also incorrect - there is no "lib" prefix when
compiling with mingw.
diff --git a/Makefile b/Makefile
index c5404a0..5b6ceca 100644
--- a/Makefile
+++ b/Makefile
@@ -375,7 +375,7 @@
 
 all: $(LIBRARY) $(ARCHIVE) $(PKGCFGF)
 ifeq (,$(findstring yes,$(CAPSTONE_BUILD_CORE_ONLY)))
-	@V=$(V) $(MAKE) -C cstool
+	@V=$(V) CC=$(CC) $(MAKE) -C cstool
 ifndef BUILDDIR
 	cd tests && $(MAKE)
 else