fcm make: inherited build: fix non-failure

This change fixes #110.

Recipe to repeat bug:
* Inherit a build.
* Override with 2 independent Fortran source files containing modules.
* Both files cause build failure. (E.g. due to syntax error.)
* Run `fcm make` once, 1st source file fails.
* Fix 1st source file.
* Run `fcm make` again. Should fail, but doesn't.