Respect user provided main-files

[awvwgk]

Fixes #643

[LKedward]

Thanks @awvwgk, looks good to me! Might be worth adding a c main test case to examples_packages for future testing? Perhaps also worth updating the wording of Executable Targets to show that c executables can be specified (not detected automatically).

[awvwgk]

Perhaps also worth updating the wording of Executable Targets to show that c executables can be specified (not detected automatically).

This can be done at the fpm-docs repo, since the manifest-reference here should be deprecated soon (#636).