`include-what-you-use` diagnostics, in practice, are specific to
the environment's compiler and standard library. Update includes
to satisfy IWYU for our CI job under Debian 12.
Introduce the function cmReadGccDepfile that parses a GCC-style depfile
and returns its content. The implementation uses a lexer that is
modeled after the re2c implementation in Ninja.
The sample files of the autotest have been created with gcc 8.3.0.
This depfile reader is to be used by the Autogen facility to make use
of the depfiles that are generated by Qt's meta object compiler.
Brad King