Added script to identify Windows version

Added script to identify the Windows version used to compile the source.
Currently the windows version for Windows NT is hard coded into the
source. By running this CMake script the _WIN32_WINNT preprocessor
variable gets set to the corresponding windows version.

CMakeModules/OsgDetermineWinVersion.cmake

@@ -0,0 +1,22 @@
+# - If Windows is used, this script sets the variable WIN32_WINNT to the corresponding windows version
+
+if (WIN32)
+  message(STATUS "Checking windows version...")
+    macro(get_WIN32_WINNT version)
+        if (CMAKE_SYSTEM_VERSION)
+            set(ver ${CMAKE_SYSTEM_VERSION})
+            string(REGEX MATCH "^([0-9]+).([0-9])" ver ${ver})
+            string(REGEX MATCH "^([0-9]+)" verMajor ${ver})
+            # Check for Windows 10, b/c we'll need to convert to hex 'A'.
+            if ("${verMajor}" MATCHES "10")
+                set(verMajor "A")
+                string(REGEX REPLACE "^([0-9]+)" ${verMajor} ver ${ver})
+            endif ("${verMajor}" MATCHES "10")
+            # Remove all remaining '.' characters.
+            string(REPLACE "." "" ver ${ver})
+            # Prepend each digit with a zero.
+            string(REGEX REPLACE "([0-9A-Z])" "0\\1" ver ${ver})
+            set(${version} "0x${ver}")
+        endif(CMAKE_SYSTEM_VERSION)
+    endmacro(get_WIN32_WINNT)
+endif(WIN32)