Even though Makefile generators pass source files and include
directories by absolute path to the compiler, the compiler may generate
depfile paths relative to the current working directory. For example,
`ccache` with `CCACHE_BASEDIR` may transform paths this way. When
reading a depfile, convert relative dependencies to absolute paths
before placing them in `compiler_depend.make`, which is later evaluated
in the top-level build directory.
Fixes: #22364
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.
