cmake: Start adding compiler flags

Only has warnings for now

Next to come is the support for developer-build and enabling Werror

Change-Id: I8070dc06eb439c2a03007cce975c8147ff7e1582
Reviewed-by: Kevin Funk <kevin.funk@kdab.com>

cmake/QtBaseGlobalTargets.cmake

@@ -64,6 +64,7 @@ install(EXPORT "${INSTALL_CMAKE_NAMESPACE}Targets" NAMESPACE ${INSTALL_CMAKE_EXP
 ## Install some QtBase specific CMake files:
 install(FILES
         cmake/QtBuild.cmake
+        cmake/QtCompilerFlags.cmake
         cmake/QtCompilerOptimization.cmake
         cmake/QtFeature.cmake
         cmake/QtPlatformSupport.cmake

cmake/QtCompilerFlags.cmake

@@ -0,0 +1,6 @@
+# Set warnings. All compilers except MSVC support -Wall -Wextra
+if (MSVC)
+    add_compile_options(/W3)
+else()
+    add_compile_options(-Wall -Wextra)
+endif()

cmake/QtSetup.cmake

@@ -42,6 +42,9 @@ include(QtFeature)
 ## Compiler optimization flags:
 include(QtCompilerOptimization)
 
+## Compiler flags:
+include(QtCompilerFlags)
+
 ## Find host tools (if non native):
 set(HOST_QT_TOOLS_DIRECTORY "" CACHE PATH "Directory with Qt host tools.")