Makefile: Allow to really disable the dynamic library

If the user disables the build of the dynamic library, we have to
link the tools with the static library. If we don't, the dependencies
will cause the dynamic library to be built regardless of the user's
request.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Fixes: 9906b2ecb6 ("Makefile: Add flag to disable dynamic library")

Makefile

@@ -43,6 +43,8 @@ endif
 ifeq ($(BUILD_DYNAMIC_LIB),0)
 ifeq ($(BUILD_STATIC_LIB),0)
 $(error BUILD_DYNAMIC_LIB and BUILD_STATIC_LIB cannot be disabled at the same time)
+else
+USE_STATIC_LIB := 1
 endif
 endif