mutate: allow configuring of compiler mapping

[joakim-brannstrom]

Problem
The option:

[compiler]
# use this compilers system includes instead of the one used in the compile_commands.json
# use_compiler_system_includes = "/path/to/c++"

overwrite all compilers used which may make it hard in a project that is a combination of both C and C++. The languages have diverged such that a C++ compiler cannot in all cases compile the C code.

Proposed Solution
Change use_compiler_system_includes to a compiler dictionary that can hold multiple compilers.