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Attached is a small project containing .c, .cpp, .m, and .mm source files where target_precompile_headers() is used to precompile a single prefix.h.

The generated Xcode project contains:

    GCC_PRECOMPILE_PREFIX_HEADER = YES
    GCC_PREFIX_HEADER            = .../CMakeFiles/Test.dir/Debug/cmake_pch.hxx

The GCC_PREFIX_HEADER header is compiled by Xcode once for each language in the project:

    clang -x c-header             -target ...
    clang -x c++-header           -target ...
    clang -x objective-c-header   -target ...
    clang -x objective-c++-header -target ...

However the cmake_pch.hxx header only includes its contents when __cplusplus is defined:

    #ifndef CMAKE_SKIP_PRECOMPILE_HEADERS
    #ifdef __cplusplus
    #include "/Users/test/Desktop/xcode_pch/prefix.h"
    #endif // __cplusplus
    #endif // CMAKE_SKIP_PRECOMPILE_HEADERS

This means that prefix.h is only visible to C++ and Objective-C++ source.

As Xcode's GCC_PREFIX_HEADER setting only takes a single path the Xcode generator should generate a single language-agnostic header:

    GCC_PRECOMPILE_PREFIX_HEADER = YES
    GCC_PREFIX_HEADER            = .../CMakeFiles/Test.dir/Debug/cmake_pch.h
 
 
    #ifndef CMAKE_SKIP_PRECOMPILE_HEADERS
    #include "/Users/test/Desktop/xcode_pch/prefix.h"
    #endif // CMAKE_SKIP_PRECOMPILE_HEADERS

Xcode will then generate per-language builds of this precompiled header itself, passing the correct build to each source file as required.

xcode_pch.tar.gz