Fix build with GCC that does not support the -Wsuggest-override option.

linphone-desktop/CMakeLists.txt

@@ -28,13 +28,15 @@ set(TARGET_NAME linphone-qt)
 set(CMAKE_CXX_STANDARD 11)
 
 include(GNUInstallDirs)
+include(CheckCXXCompilerFlag)
 
 # Use automatically moc from Qt5.
 set(CMAKE_AUTOMOC ON)
 set(CMAKE_INCLUDE_CURRENT_DIR ON)
 
 if(NOT WIN32)
-set(CUSTOM_FLAGS "\
+  check_cxx_compiler_flag("-Wsuggest-override" SUGGEST_OVERRIDE)
+  set(CUSTOM_FLAGS "\
 -Wall \
 -Wcast-align \
 -Wconversion \
@@ -47,10 +49,12 @@ set(CUSTOM_FLAGS "\
 -Wold-style-cast \
 -Woverloaded-virtual \
 -Wpointer-arith \
--Wsuggest-override \
 -Wuninitialized \
 -Wunused \
 ")
+  if(SUGGEST_OVERRIDE)
+    set(CUSTOM_FLAGS "${CUSTOM_FLAGS} -Wsuggest-override")
+  endif()
 endif()
 set(CMAKE_CXX_FLAGS  "${CMAKE_CXX_FLAGS} ${CUSTOM_FLAGS}")
 set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -DQT_QML_DEBUG -DQT_DECLARATIVE_DEBUG")