Cannot build with clang-cl

[sykhro]

Note: for general questions and comments, please use the forums at:
https://groups.google.com/g/json-c

Describe the bug
Building with clang-cl currently fails due to CMake not distinguishing it from clang with the GNU-style CLI.

clang-cl.exe  /nologo -D_CRT_NONSTDC_NO_WARNINGS -D_CRT_SECURE_NO_WARNINGS -D_GNU_SOURCE -D_UNICODE -IC:\Users\Erin\.conan2\p\b\json-e2cfe01b7c40f\b\src -IC:\Users\Erin\.conan2\p\b\json-e2cfe01b7c40f\b\build\Debug /EHsc /permissive- /Zc:preprocessor /volatile:iso -Wno-unused-command-line-argument -fuse-ld=lld /DWIN32 /D_WINDOWS /W3 -ffunction-sections -fdata-sections -Werror -Wall -Wcast-qual -Wno-error=deprecated-declarations -Wextra -Wwrite-strings -Wno-unused-parameter /Zi /Ob0 /Od /RTC1 -MDd /showIncludes /FoCMakeFiles\json-c.dir\json_util.c.obj /FdCMakeFiles\json-c.dir\json-c.pdb -c -- C:\Users\Erin\.conan2\p\b\json-e2cfe01b7c40f\b\src\json_util.c
clang-cl: error: unknown argument ignored in clang-cl: '-ffunction-sections' [-Werror,-Wunknown-argument]
clang-cl: error: unknown argument ignored in clang-cl: '-fdata-sections' [-Werror,-Wunknown-argument]

The proper fix is using CMAKE_C_COMPILER_FRONTEND_VARIANT, but it would require bumping the CMake version to 3.14.

I know about the DISABLE_WERROR option, but clang-cl needs to be detected as MSVC.

Steps To Reproduce

1. Configure CMake with CMAKE_C_COMPILER set to clang-cl
2. Build

Version and Platform

• json-c version: >= 0.16
• OS: Any
• Custom cmake/build flags, if any: None that would be relevant

[hawicz]

I don't think bumping up to 3.14 is something we can do just yet.
I found https://discourse.cmake.org/t/how-to-determine-if-clang-or-clang-cl-is-used-on-windows/4129/9 as an example of using execute_process(COMMAND "${CMAKE_${lang}_COMPILER}" -?) to figure out what kind of clang is in use, so we could stuff something like that into json-c's CMakeLists.txt

[gitartpiano]

The simple workaround which I have described at #843 might work for you.

[hawicz]

I don't have an environment to test it in, but commit e3464a2 should make this work better.