BUILD: Allow overwrite of cpp std & target platform

If CMAKE_CXX_STANDARD and/or CMAKE_OSX_DEPLOYMENT_TARGET are specified
on the command line when invoking cmake, these values will be respected
instead of being overwritten by the project.

CMakeLists.txt

@@ -24,6 +24,15 @@ if ("${BUILD_NUMBER}" STREQUAL "")
 	endif()
 endif()
 
+if(NOT DEFINED CMAKE_CXX_STANDARD)
+	set(CMAKE_CXX_STANDARD 14)
+endif()
+set(CMAKE_CXX_EXTENSIONS OFF)
+
+if(NOT DEFINED CMAKE_OSX_DEPLOYMENT_TARGET)
+	set(CMAKE_OSX_DEPLOYMENT_TARGET 10.13)
+endif()
+
 # Get compilation year
 string(TIMESTAMP MUMBLE_BUILD_YEAR "%Y")
 
@@ -37,10 +46,6 @@ project(mumble
 set(3RDPARTY_DIR "${CMAKE_SOURCE_DIR}/3rdparty")
 set(PLUGINS_DIR "${CMAKE_SOURCE_DIR}/plugins")
 
-set(CMAKE_CXX_STANDARD 14)
-set(CMAKE_CXX_EXTENSIONS OFF)
-set(CMAKE_OSX_DEPLOYMENT_TARGET 10.13)
-
 list(APPEND CMAKE_MODULE_PATH
 	"${CMAKE_SOURCE_DIR}/cmake"
 	"${CMAKE_SOURCE_DIR}/cmake/FindModules"