Split clang options passed to clang, to be more concise and less messy. Start splitting C and C++ options.
Add option valid-abspath to add absolute path, which should be considered valid.
Add test for warning about abspath and selectively disabling it.
Add valid_abspath option to emcc --help.
Apply requested formatting changes.
Merge pull request #2232 from abergmeier/empkg_prerequisites

Empkg prerequisites