Cmake build: Detect clang and adapt definition of STD_STRING_USES_COW.

CMakeLists.txt

@@ -259,13 +259,10 @@ if(UNIX OR MINGW)
 		endif()
 		set(LYX_GCC11_MODE "${CXX11_FLAG}")
 	endif()
-	set(STD_STRING_USES_COW 1)
 else()
 	if(MSVC10)
 		set(LYX_USE_TR1 1)
 		#set(LYX_USE_TR1_REGEX 1) #TODO should we use it in ECMAScript mode?
-	else()
-		set(STD_STRING_USES_COW 1)
 	endif()
 endif()
 

development/cmake/ConfigureChecks.cmake

@@ -126,6 +126,17 @@ check_cxx_source_compiles(
 	"
 LYX_CALLSTACK_PRINTING)
 
+get_filename_component(cxx_base "${CMAKE_CXX_COMPILER}" NAME_WE)
+if(cxx_base MATCHES "^clang(-[0-9]+\\.[0-9]+\\.[0-9]+)?(\\+\\+)?$")
+	set(STD_STRING_USES_COW)
+else()
+	if("${MINGW}")
+		set(STD_STRING_USES_COW)
+	else()
+		set(STD_STRING_USES_COW 1)
+	endif()
+endif()
+
 if(LYX_USE_QT MATCHES "QT5")
   if (Qt5X11Extras_FOUND)
     get_target_property(_x11extra_prop Qt5::X11Extras IMPORTED_CONFIGURATIONS)

development/cmake/config.h.cmake

@@ -60,6 +60,9 @@
 // Define if callstack can be printed
 #cmakedefine LYX_CALLSTACK_PRINTING 1
 
+// Define if std::string uses copy-on-write
+#cmakedefine STD_STRING_USES_COW 1
+
 #cmakedefine Z_PREFIX 1
 
 ${Include_used_spellchecker}