Fix `clang-cl` `MSVC` frontend received `GNU` flag on assembling

[Challanger524]

PR for #293

Put if (GNU/Clang) into the MASM's else clause since Clang-CL validates positive at CMAKE_CXX_COMPILER_ID STREQUAL "Clang" while also having MSVC frontend for compile options and utilising ML masm assembler.

UPD: clang-cl /quiet support has been dropped due to inability to know real version for sure. Side (weird) fix for /quiet flag being passed on MSVC_VERSION: 1936 despite Visual Studio Enterprise 2019 Version 16.11.35:

Dropping context

• Visual Studio Enterprise 2019 Version 16.11.35
• (Microsoft (R) Macro Assembler (x64) Version 14.29.30154.0)
• message(STATUS "MSVC_VERSION: ${MSVC_VERSION}") = -- MSVC_VERSION: 1936 report
• despite doc says that: /quiet Suppresses 'Assembling' message. Available in Visual Studio 17.6 and later. Looks like CMake gives wrong MSVC_VERSION for clang-cl.

Tested

On MS Windows with CMake with next compilers: MSVC, msys2/mingw64/GCC, msys2/clang64/Clang, Clang-CL (chocolatey install)

[Challanger524]

@marakew can you copy patch to your boost::context and validate?

[marakew]

seems works

[marakew]

@olk how about merge ?

[olk]

ty