CMake spec2def improve

[HBelusca]

Purpose

Do some simple cmake files cleanup, and improve some aspects of spec2def() function.

Proposed changes

Describe what you propose to change/add/fix with this pull request.

• Some cmake files cleanup:

[CMAKE] Get rid of the macro_mc() macro.

Since COMMAND_MC is the same for both MSVC and GCC, get rid of the
macro_mc() macro and directly use said command in add_message_headers().

[CMAKE] MSVC's "SPEC2DEF_ARCH" == GCC's "ARCH2", so use a common name instead. Simplify its initialization. Move __x86_64 to global CMakeLists.txt.

[CMAKE] Simplify generate_import_lib() code.

- Simplify by storing the implib def file name in a variable, as done in msvc.cmake.
- Remove deprecated STATIC_LIBRARY_FLAGS usage.

• Improve cmake-file spec2def() function:

[CMAKE] Move spec2def() function common implementation into CMakeMacros.cmake.

The slight different spec2def invocation command definition is stored
in a COMMAND_SPEC2DEF variable, used by both generate_import_lib() and
spec2def() functions.

[CMAKE] Allow the spec2def() function to take a list of .spec files (more than one).

When more than one .spec file are present, they get concatenated
together and the resulting file is passed to spec2def.