CMake: Fix headersclean to work on macOS

It appears there's a difference between the chosen macOS compiler path
in the CI versus the compiler path on my local machine.

In the CI the chosen compiler path ends up 'being /usr/bin/clang++'
whereas for me locally it's

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++

For some reason the headersclean commands succeed in the CI, but
locally they fail for me saying that standard library includes can not
be found, unless an explicit sysroot flag is specified.

I assume that in the CI the '/usr/bin/clang++' compiler shim chooses some
implcit sysroot, whereas the longer Xcode compiler expects an explicit
sysroot.
It's probably also affected by the fact that in the CI we pass an
explicit CMAKE_OSX_SYSROOT to a non-standard Xcode location e.g.

/Applications/Xcode11.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk

Note the '11' in the Xcode app name.

Locally I don't pass a custom CMAKE_OSX_SYSROOT, and my Xcode is
installed in a regular location e.g /Applications/Xcode.app.

The sysroot flag and path is added to regular CXX compilation rules
inside CMake's core (in cmLocalGenerator.cxx).

Reuse the same variables that CMake uses and add them to our headersclean
command rules.

Task-number: QTBUG-82615
Change-Id: Ic03ea27e39471f5fa168eb5970bf3d3f1d1be251
Reviewed-by: Cristian Adam <cristian.adam@qt.io>

cmake/QtHeadersClean.cmake

@@ -83,6 +83,12 @@ function(qt_internal_add_headers_clean_target
         # Use strict mode C++17, with no GNU extensions (see -pedantic-errors above).
         list(APPEND hcleanFLAGS -std=c++17)
 
+        set(cxx_flags ${CMAKE_CXX_FLAGS})
+
+        if(APPLE AND CMAKE_OSX_SYSROOT)
+            list(APPEND cxx_flags "${CMAKE_CXX_SYSROOT_FLAG}" "${CMAKE_OSX_SYSROOT}")
+        endif()
+
         foreach(header ${hclean_headers})
             get_filename_component(input_path "${header}" ABSOLUTE)
             set(artifact_path "header_${header}.o")
@@ -90,7 +96,7 @@ function(qt_internal_add_headers_clean_target
             add_custom_command(
                 OUTPUT "${artifact_path}"
                 COMMENT "headersclean: Checking header ${header}"
-                COMMAND "${CMAKE_CXX_COMPILER}" -c ${CMAKE_CXX_FLAGS} ${hcleanFLAGS}
+                COMMAND "${CMAKE_CXX_COMPILER}" -c ${cxx_flags} ${hcleanFLAGS}
                 -I "${QT_BUILD_DIR}/include" -I "${CMAKE_INSTALL_PREFIX}/include"
                 ${hcleanDEFS} -xc++ "${input_path}"
                 -o${artifact_path}