[C] do not generate code without main reactor

[housengw]

Do not generate code when there is no main reactor

Fixes #913

[lhstrh]

How does this PR relate to #913? Not generating code when there is no main reactor means that we cannot generate any pure library code. Is that really what we want?

[housengw]

The error outlined in #913 is due to CGenerator not checking if main reactor exist before generating code in doGenerate.

The Cpp target also doesnt generate code when there is no main reactor. It might make sense to standardize code generation behavior across targets.

[cmnrd]

I think we should really generate a library if there is no main reactor. In fact, I think we should create a library for all LF files and only generate a small binary that uses this library if there is a main reactor. Note that for C++ we have a long open issue: #83.

For the short term solution, though, it might be better to just forbid compilation without a main reactor. Without a package system, it does not make much sense to compile LF programs as libraries.

[lhstrh]

Looks like a good quick fix. Like @cmnrd said, more work would be needed for the generation of library code. I've made suggestions for the removal of checks that I think are unnecessary.

[lhstrh]

Oh, and congrats on filing PR #1000 🥳

[lhstrh]

One more suggestion.