Setting -Wall on compilers that support it.

src/CMakeLists.txt

@@ -8,6 +8,22 @@ set (CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/Modules/"
 # locations on all platforms.
 include(GNUInstallDirs)
 
+include(CheckCXXCompilerFlag)
+
+function(enable_cxx_compiler_flag_if_supported flag)
+    string(FIND "${CMAKE_CXX_FLAGS}" "${flag}" flag_already_set)
+    if(flag_already_set EQUAL -1)
+        check_cxx_compiler_flag("${flag}" flag_supported)
+        if(flag_supported)
+            set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${flag}" PARENT_SCOPE)
+        endif()
+        unset(flag_supported CACHE)
+    endif()
+endfunction()
+
+
+enable_cxx_compiler_flag_if_supported("-Wall")
+
 set(CMAKE_CXX_EXTENSIONS OFF)
 # set(CMAKE_CXX_STANDARD 14)
 # set(CMAKE_CXX_STANDARD_REQUIRED ON)