Overzealous `/permissive-` breaks cross-compiled builds

[nyoma-diamond]

Currently, the usage of target_compile_options(fast_float INTERFACE /permissive-) when the host compiler allows /permissive (as validated by CHECK_CXX_COMPILER_FLAG(/permissive- FASTFLOAT_COMPILER_SUPPORTS_PERMISSIVE)) is overzealous and breaks cross-compiled builds. For example, if the host is built using MSVC, but the downstream uses any non-MSVC compiler, compiling will fail with an error such as this:

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

This is an extension of #304/#305, which fixed a different edge of this issue.

With the current approach (using target_compile_options(fast_float INTERFACE /permissive-)) the compiler flag gets propagated forward into the generated fast_floats-targets.cmake at build-time:

set_target_properties(FastFloat::fast_float PROPERTIES
  INTERFACE_COMPILE_FEATURES "cxx_std_11"
  INTERFACE_COMPILE_OPTIONS "/permissive-"
  INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include"
)

Which erroneously forces the consumer to use the /permissive- flag even if it's not relevant for the downstream compiler.

This issue is particularly relevant when working with fast_float via vcpkg, as vcpkg makes it very easy to set up cross-compiling by building for the host development system with their compiler of choice, and then compile for the target using a different relevant compiler, allowing development for other OS's or to allow special compiler-specific features to be enabled/disabled at build time (in my case, my target builds are done using Clang++ as I need some of its features, but my host uses MSVC for speed and ease of setup).

[lemire]

Thanks for the comments. I invite a pull request.