Remove macOS 10.5 detection

This also gets rid of exec_program which has been deprecated in
CMake.

cmake/Config.cmake

@@ -19,14 +19,6 @@ elseif(${CMAKE_SYSTEM_NAME} MATCHES "FreeBSD")
     set(SFML_OS_FREEBSD 1)
 elseif(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
     set(SFML_OS_MACOSX 1)
-
-    # detect OS X version. (use '/usr/bin/sw_vers -productVersion' to extract V from '10.V.x'.)
-    EXEC_PROGRAM(/usr/bin/sw_vers ARGS -productVersion OUTPUT_VARIABLE MACOSX_VERSION_RAW)
-    STRING(REGEX REPLACE "10\\.([0-9]+).*" "\\1" MACOSX_VERSION "${MACOSX_VERSION_RAW}")
-    if(${MACOSX_VERSION} LESS 5)
-        message(FATAL_ERROR "Unsupported version of OS X : ${MACOSX_VERSION_RAW}")
-        return()
-    endif()
 else()
     message(FATAL_ERROR "Unsupported operating system")
     return()