Precompiled header support for Clang

[stephenberry]

          > Precompiled headers helps a lot with Glaze.

When i try to use them with clang on glaze I only get errors building pch

if(${CMAKE_VERSION} VERSION_GREATER_EQUAL "3.16.0")
file(GLOB_RECURSE HEADERS
"${CMAKE_SOURCE_DIR}/include/.h"
"${CMAKE_SOURCE_DIR}/include/.hpp"
)
target_precompile_headers( glaze_glaze INTERFACE ${HEADERS})
endif()

like that

Originally posted by @arturbac in #763 (comment)

[stephenberry]

We should probably also add precompile header testing to the GitHub actions.

[stephenberry]

I'm converting this to a Discussion. I will gladly accept pull requests that add this feature, but I'm not going to focus on it, because I'd rather focus these kinds of efforts on C++20 modules support, which will supersede precompiled headers in the future.