GzSetCompilerFlags: Fix detection of clang-cl

[traversaro]

🦟 Bug fix

Fixes compilation of gz-* projects with clang-cl.

Summary

On Windows, there are two drivers available for the clang compiler:

• clang, that takes in input GCC-style compiler flags
• clang-cl, that takes in input MSVC-style compiler flags

When clang-cl is used, it does not make sense to pass to it GCC-style compiler flags. Befor this PR, GCC-style compiler flags were passed to clang-cl, that due to some combination with the existing MSVC flags resulted in the wrong C++ standard version be used when compiling the project, creating an endless list of warnings like:

C:\src\gz-math\include\gz/math/Vector2.hh(34,5): warning: inline namespaces are incompatible with C++98 [-Wc++98-compat]
    inline namespace GZ_MATH_VERSION_NAMESPACE {
    ^
C:\src\gz-math\include\gz/math/Vector2.hh(51,15): warning: 'constexpr' specifier is incompatible with C++98 [-Wc++98-compat]
      public: constexpr Vector2()
              ^
C:\src\gz-math\include\gz/math/Vector2.hh(59,15): warning: 'constexpr' specifier is incompatible with C++98 [-Wc++98-compat]
      public: constexpr Vector2(const T &_x, const T &_y)
              ^
C:\src\gz-math\include\gz/math/Vector2.hh(66,47): warning: defaulted function definitions are incompatible with C++98 [-Wc++98-compat]
      public: Vector2(const Vector2<T> &_v) = default;
                                              ^
C:\src\gz-math\include\gz/math/Vector2.hh(69,28): warning: defaulted function definitions are incompatible with C++98 [-Wc++98-compat]
      public: ~Vector2() = default;
                           ^
C:\src\gz-math\include\gz/math/Vector2.hh(227,55): warning: defaulted function definitions are incompatible with C++98 [-Wc++98-compat]
      public: Vector2 &operator=(const Vector2 &_v) = default;
                                                      ^
C:\src\gz-math\include\gz/math/Vector2.hh(52,13): warning: generalized initializer lists are incompatible with C++98 [-Wc++98-compat]
      : data{0, 0}
            ^
C:\src\gz-math\include\gz/math/Vector2.hh(60,13): warning: generalized initializer lists are incompatible with C++98 [-Wc++98-compat]
      : data{_x, _y}

This PR fixes this by only adding GCC-style compiler flags if clang is used, and not adding them if clang-cl is using. Strictly speaking the variable used to check if clang-cl is used is CMAKE_CXX_COMPILER_FRONTEND_VARIANT, that was only introduced in CMake 3.14 while gz-cmake3 supprots CMake 3.2 . However, if that code is evaluted in CMake <= 3.13, the variable CMAKE_CXX_COMPILER_FRONTEND_VARIANT will be empty and the check:

CMAKE_CXX_COMPILER_FRONTEND_VARIANT STREQUAL "MSVC"

will be false.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[scpeters]

thanks for the fix!