add support for scons cache and enable it for appveyor

.appveyor.yml

@@ -1,7 +1,9 @@
 os: Visual Studio 2015
 
 environment:
+  HOME: "%HOMEDRIVE%%HOMEPATH%"
   PYTHON: C:\Python27
+  SCONS_CACHE: "%HOME%\\scons_cache"
   matrix:
     - VS: C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat
       GD_PLATFORM: windows
@@ -9,6 +11,9 @@ environment:
       TARGET: release_debug
       ARCH: amd64
 
+cache:
+  - "%SCONS_CACHE%"
+
 install:
   - SET "PATH=%PYTHON%;%PYTHON%\\Scripts;%PATH%"
   - pip install --egg scons  # it will fail on AppVeyor without --egg flag

SConstruct

@@ -434,6 +434,11 @@ if selected_platform in platform_list:
     if (True): # FIXME: detect GLES3
         env.Append( BUILDERS = { 'GLES3_GLSL' : env.Builder(action = methods.build_gles3_headers, suffix = 'glsl.gen.h',src_suffix = '.glsl') } )
 
+    scons_cache_path = os.environ.get("SCONS_CACHE")
+    if scons_cache_path != None:
+        CacheDir(scons_cache_path)
+        print("Scons cache enabled... (path: '" + scons_cache_path + "')")
+
     Export('env')
 
     # build subdirs, the build order is dependent on link order.