CxxUtils+Projects: add checkergcc configuration

Improvements to the checkergcc plugins configuration:

• Split the checkergcc configuration into a "base" configuration and build specific configurations. The "base" config file contains settings that are always used. (cc @ssnyder)
• Add CxxUtilsSettings module that sets up the ATLAS_GCC_CHECKERS_CONFIG variable with the appropriate configuration files depending on the build type. Call this module from those projects that actually build the checkergcc plugins.
• For WorkDir use the configuration files from a local source tree if available, otherwise from the parent release.
• Note that the *.config files are no longer installed. We just use them directly from the installed src directory, which is always available.

Relates to ATLINFR-4570.