Makefile.nmake: be more robust when cmake is missing.

Previously, when cmake was missing, the build dir would
be created anyway, and subsequent attempts (even with cmake)
would fail, because cmake would not be run.

Depending on build/CMakeFiles is more robust -- this
won't be created unless cmake is run.

Partially addresses #85.

Makefile.nmake

@@ -6,10 +6,10 @@ SPEC=test/spec.txt
 PROG=$(BUILDDIR)\src\cmark.exe
 GENERATOR=NMake Makefiles
 
-all: $(BUILDDIR)
+all: $(BUILDDIR)/CMakeFiles
 	@cd $(BUILDDIR) && $(MAKE) /nologo && cd ..
 
-$(BUILDDIR):
+$(BUILDDIR)/CMakeFiles:
 	@-mkdir $(BUILDDIR) 2> nul
 	cd $(BUILDDIR) && \
 	cmake \