make CMAKE_OSX_DEPLOYMENT_TARGET a cache variable

Allow users to override the macOS deployment target via cmake
command line or cache configuration.

Author: azuwis

CMakeLists.txt

@@ -25,7 +25,7 @@ message(STATUS "building for ${CMAKE_SYSTEM_NAME}")
 
 if (${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
 	set(USB_BACKEND_DARWIN TRUE)
-	set(CMAKE_OSX_DEPLOYMENT_TARGET 10.13)
+	set(CMAKE_OSX_DEPLOYMENT_TARGET 10.13 CACHE STRING "Minimum OS X deployment version")
 	find_program(OTOOL_BIN otool)
 endif()