Gcc does not support `x c*-header` when using `-` as input filename,
which is what ALE does.
Rework the feature to only use `-x c*-header` flag when using Clang and
not GCC.
The feature is now also controlled with the variable
`g:ale_c_cc_use_header_lang_flag` and
`g:ale_cpp_cc_use_header_lang_flag`.
When linting an header file in C or C++, `-x c-header` or
`-x c++-header` should be used instead of `-x c` or `-x c++`.
Using `-x c` or `-x c++` for headers files can lead to unused variables
and functions marked as static inlined as seen in #4096.
Using `-x c-header` or `-x c++-header` solve these issues.
The list of file extensions that are considered as header files can be
configured with the variables `g:ale_c_cc_header_exts` and
`g:ale_cpp_cc_header_exts`.
Users can easily be confused when they set some options for a C or C++
compiler, and another compiler is run with different options, which
still reports errors. To remedy this, the existing `gcc` and `clang`
linters have been replaced with a `cc` linter that will run either
compiler.
This is a breaking change for ALE v3.0.0.
Nicolas Pauss
w0rp