Makefile tweak for better subproject handling

Don't include the makefile fragments of subprojects that aren't built.

With this the build fails more reasonably when SUBS is set incorrectly.
1 file changed
tree: f7e234004e26109b1c8b8d3dd2653e4632a03a31
  1. math/
  2. string/
  3. .gitignore
  4. config.mk.dist
  5. LICENSE
  6. Makefile
  7. README