Support configure -platform linux-clang-libc++

Detect libc++ config and add appropriate compiler and linker flags.

Pick-to: 6.3
Change-Id: I9ec91b3ace987599d4e79e43b1b75aa67cd5caeb
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

cmake/QtFeature.cmake

@@ -943,6 +943,11 @@ function(qt_config_compile_test name)
 
             set(CMAKE_REQUIRED_FLAGS ${arg_COMPILE_OPTIONS})
 
+            # Pass -stdlib=libc++ on if necessary
+            if (INPUT_stdlib_libcpp OR QT_FEATURE_stdlib_libcpp)
+                list(APPEND CMAKE_REQUIRED_FLAGS "-stdlib=libc++")
+            endif()
+
             # For MSVC we need to explicitly pass -Zc:__cplusplus to get correct __cplusplus
             # define values. According to common/msvc-version.conf the flag is supported starting
             # with 1913.
@@ -1003,6 +1008,15 @@ function(qt_get_platform_try_compile_vars out_var)
     list(APPEND flags "CMAKE_C_STANDARD")
     list(APPEND flags "CMAKE_CXX_STANDARD")
 
+    # Pass -stdlib=libc++ on if necessary
+    if (INPUT_stdlib_libcpp OR QT_FEATURE_stdlib_libcpp)
+        if(CMAKE_CXX_FLAGS)
+            string(APPEND CMAKE_CXX_FLAGS " -stdlib=libc++")
+        else()
+            set(CMAKE_CXX_FLAGS "-stdlib=libc++")
+        endif()
+    endif()
+
     # Assemble the list with regular options.
     set(flags_cmd_line "")
     foreach(flag ${flags})

cmake/QtPlatformTargetHelpers.cmake

@@ -26,6 +26,11 @@ function(qt_internal_setup_public_platform_target)
         target_link_libraries(Platform INTERFACE log)
     endif()
 
+    if (QT_FEATURE_stdlib_libcpp)
+        target_compile_options(Platform INTERFACE "-stdlib=libc++")
+        target_link_options(Platform INTERFACE "-stdlib=libc++")
+    endif()
+
     qt_set_msvc_cplusplus_options(Platform INTERFACE)
 
     # Propagate minimum C++ 17 via Platform to Qt consumers (apps), after the global features

cmake/QtProcessConfigureArgs.cmake

@@ -733,6 +733,9 @@ function(guess_compiler_from_mkspec)
     if(guess_cxx_compiler AND NOT cxx_compiler STREQUAL "")
         push("-DCMAKE_CXX_COMPILER=${cxx_compiler}")
     endif()
+    if(mkspec MATCHES "-libc\\+\\+$")
+        push("-DINPUT_stdlib_libcpp=ON")
+    endif()
     set(cmake_args "${cmake_args}" PARENT_SCOPE)
 endfunction()
 

configure.cmake

@@ -849,6 +849,11 @@ qt_feature("zstd" PRIVATE
     LABEL "Zstandard support"
     CONDITION ZSTD_FOUND
 )
+qt_feature("stdlib-libcpp" PRIVATE
+    LABEL "Using stdlib=libc++"
+    AUTODETECT OFF
+    CONDITION LINUX AND NOT ANDROID
+)
 # special case begin
 # Check whether CMake was built with zstd support.
 # See https://gitlab.kitware.com/cmake/cmake/-/issues/21552