[cbuild2cmake] Trigger project executes after project build step

[EaselinkBachmann]

This PR adds a new CONTEXT-executes target to CMake that is automatically run after build that triggers the executes statements that depend on the build outputs of that project.

Fixes #185.

This is the local fix that I described in cbuild#244. I'm not sure if this is exactly how you intend on resolving this, but I'm providing my patch in the hope that it will be useful.

[EaselinkBachmann]

Fixed to also correctly work with solution level executes. Each execute statement is now added to the ${CONTEXT}-executes target of each context in the execute statement's depends-on: key.

Currently doesn't deal with multiple dependent executes statements though (i.e., an executes-A depends on executes-B depends on context-X currently leads to executes-A not being automatically run). Handling these kinds of transitive reverse-dependencies is difficult.

[brondani]

I'm not sure if this is exactly how you intend on resolving this, but I'm providing my patch in the hope that it will be useful.

Actually my proposal was slightly different, I was thinking on fully replacing the context-level "post-build" entries add_custom_target/add_custom_command by ExternalProject_Add_Step, but I like your solution.
Thanks for this contribution!

Currently doesn't deal with multiple dependent executes statements though (i.e., an executes-A depends on executes-B depends on context-X currently leads to executes-A not being automatically run). Handling these kinds of transitive reverse-dependencies is difficult.

Possibly this could be solved by iterating again over the depends-on until no further dependency is found. We can address that in a second moment.