Compile error on configuration on MacOS

[dforsten]

On MacOS I am getting these errors on executing conan_cmake_run

-- Conan: Automatic detection of conan settings from cmake
In file included from <stdin>:1:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/string:504:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/string_view:175:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/__string:56:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/algorithm:641:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/cstring:60:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/string.h:60:15: fatal error: 'string.h' file not found
#include_next <string.h>
              ^~~~~~~~~~
1 error generated.

The errors are caused by this command:

    execute_process(
        COMMAND ${CMAKE_COMMAND} -E echo "#include <string>"
        COMMAND ${CMAKE_CXX_COMPILER} -x c++ ${compile_options} -E -dM -
        OUTPUT_VARIABLE string_defines
    )

Where ${CMAKE_CXX_COMPILER} is expanding to /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++. Unlike for /usr/bin/c++ the sysroot is not set when executing that binary, but setting the sysroot manually fixes the compile issue:

    execute_process(
        COMMAND ${CMAKE_COMMAND} -E echo "#include <string>"
        COMMAND ${CMAKE_CXX_COMPILER} -x c++ --sysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk ${compile_options} -E -dM -
        OUTPUT_VARIABLE string_defines
    )

[dforsten]

Pull Request with a fix submitted.

[dforsten]

The compile error is present in the Travis CI logs for XCode 10 and 11 in builds before this fix.

The builds do not fail, but having to scroll past these compile errors on every CMake run on MacOS is definitely annoying.