Enforce VS2013/2015 requirement on Windows.

CMakeLists.txt

@@ -143,12 +143,8 @@ if (MSVC AND MSVC_3RDPARTY_ROOT)
       set( OSG_MSVC ${OSG_MSVC}140 )
   elseif (${MSVC_VERSION} EQUAL 1800)
       set( OSG_MSVC ${OSG_MSVC}120 )
-  elseif (${MSVC_VERSION} EQUAL 1700)
-      message(WARNING "Visual Studio 2013 will be required soon, please upgrade")
-      set( OSG_MSVC ${OSG_MSVC}110 )
-  elseif (${MSVC_VERSION} EQUAL 1600)
-      message(WARNING "Visual Studio 2013 will be required soon, please upgrade")
-      set( OSG_MSVC ${OSG_MSVC}100 )
+  else ()
+      message(FATAL_ERROR "Visual Studio 2013/2015 is required now")
   endif ()
   if (CMAKE_CL_64)
       set( OSG_MSVC ${OSG_MSVC}-64 )
@@ -227,7 +223,7 @@ else()
          int main() { return 0; }"
          SIMGEAR_OSG_USE_UTF8_FILENAME)
       if (NOT SIMGEAR_OSG_USE_UTF8_FILENAME)
-        message(WARNING "Please rebuild OSG with OSG_USE_UTF8_FILENAME set to ON")
+        message(FATAL_ERROR "Please rebuild OSG with OSG_USE_UTF8_FILENAME set to ON")
       endif()
     endif()
 endif(SIMGEAR_HEADLESS)
@@ -379,7 +375,7 @@ if(WIN32)
     if(MSVC)
         set(MSVC_FLAGS "-DWIN32 -DNOMINMAX -D_USE_MATH_DEFINES -D_CRT_SECURE_NO_WARNINGS -D__CRT_NONSTDC_NO_WARNINGS /MP")
 
-        if (NOT OSG_FSTREAM_EXPORT_FIXED AND ${MSVC_VERSION} GREATER 1599)
+        if (NOT OSG_FSTREAM_EXPORT_FIXED)
           message(STATUS "For better linking performance, use OSG with patched fstream header")
           # needed to avoid link errors on multiply-defined standard C++
           # symbols. Suspect this may be an OSG-DB export bug