Fix make dir/file not rebuilding existing files

Running `make library/foo`, `make programs/foo` or `make tests/foo` only
rebuilt the given target if it was not an existing file, because the
toplevel makefile does not know the file's dependencies and thus thought
that every such target had empty dependencies. Fix this by always invoking
make recursively.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

Makefile

@@ -46,11 +46,14 @@ tests: lib mbedtls_test
 mbedtls_test:
 	$(MAKE) -C tests mbedtls_test
 
-library/%:
+.PHONY: FORCE
+FORCE:
+
+library/%: FORCE
 	$(MAKE) -C library $*
-programs/%:
+programs/%: FORCE
 	$(MAKE) -C programs $*
-tests/%:
+tests/%: FORCE
 	$(MAKE) -C tests $*
 
 .PHONY: generated_files