Add version check logic for macOS deployment target build option.

[skip appveyor] [ci only: OSX]

CMake/Modules/UrhoCommon.cmake

@@ -96,14 +96,18 @@ elseif (TVOS)
     unset (CMAKE_OSX_DEPLOYMENT_TARGET CACHE)
 elseif (XCODE)
     set (CMAKE_OSX_SYSROOT macosx)    # Set Base SDK to "Latest OS X"
-    if (NOT CMAKE_OSX_DEPLOYMENT_TARGET)
+    if (CMAKE_OSX_DEPLOYMENT_TARGET)
+        if (CMAKE_OSX_DEPLOYMENT_TARGET VERSION_LESS 10.9)
+            message (FATAL_ERROR "The minimum supported CMAKE_OSX_DEPLOYMENT_TARGET is 10.9.")
+        endif ()
+    else ()
         # If not set, set to current running build system OS version by default
         execute_process (COMMAND sw_vers -productVersion OUTPUT_VARIABLE CURRENT_OSX_VERSION OUTPUT_STRIP_TRAILING_WHITESPACE)
         string (REGEX REPLACE ^\([^.]+\\.[^.]+\).* \\1 CMAKE_OSX_DEPLOYMENT_TARGET ${CURRENT_OSX_VERSION})
-        set (CMAKE_OSX_DEPLOYMENT_TARGET ${CMAKE_OSX_DEPLOYMENT_TARGET} CACHE STRING "Specify macOS deployment target (macOS platform only); default to current running macOS if not specified, the minimum supported target is 10.5 due to constraint from SDL library")
+        set (CMAKE_OSX_DEPLOYMENT_TARGET ${CMAKE_OSX_DEPLOYMENT_TARGET} CACHE STRING "Specify macOS deployment target (macOS platform only); default to current running macOS if not specified, the minimum supported target is 10.9")
     endif ()
     if (DEPLOYMENT_TARGET_SAVED AND NOT CMAKE_OSX_DEPLOYMENT_TARGET STREQUAL DEPLOYMENT_TARGET_SAVED)
-        set (CMAKE_OSX_DEPLOYMENT_TARGET ${DEPLOYMENT_TARGET_SAVED} CACHE STRING "Specify macOS deployment target (macOS platform only); default to current running macOS if not specified, the minimum supported target is 10.5 due to constraint from SDL library" FORCE)
+        set (CMAKE_OSX_DEPLOYMENT_TARGET ${DEPLOYMENT_TARGET_SAVED} CACHE STRING "Specify macOS deployment target (macOS platform only); default to current running macOS if not specified, the minimum supported target is 10.9" FORCE)
         message (FATAL_ERROR "CMAKE_OSX_DEPLOYMENT_TARGET cannot be changed after the initial configuration/generation. "
             "Auto reverting to its initial value. If you wish to change it then the build tree would have to be regenerated from scratch.")
     endif ()