CI: Reintroduce gcc jobs
Some of the programs (e.g. solution01-host) don't (well, shouldn't) require pacxx. Compiling and testing them with gcc should help find headers that unneccessarily include pacxx headers or use other pacxx functionality.
TODO:
-
reintroduce gcc ci job (will fail initially) -
suppress building pacxx-requiring executables when pacxx is missing -
skip pacxx-requiring unittests