fix /permissive- flag causing a compile error on clang for windows

[dankmeme01]

When using the library with Clang on Windows (not clang-cl and not MSYS2), an error is thrown by the compiler:

clang: error: no such file or directory: '/permissive-'

This is caused by these lines in CMakeLists.txt:

if(MSVC_VERSION GREATER 1910)
  target_compile_options(fast_float INTERFACE /permissive-)
endif()

Clang on Windows defines MSVC_VERSION, but does not support MSVC-like compiler flags. This PR adds an extra check to make sure the flag is only added when the used compiler supports it.

Presumably fixes #304, although it seems that CI only has MSYS2 Clang and MSVC Clang-cl workflows, neither of which are impacted by this issue, so I've only verified that it works on my machine.

[lemire]

Running tests.

[lemire]

This will be part of the next release.