scons compiling fix:

When "svnversion" can't be found (as is usual for anyone running TortoiseSVN), build_revision info will be "", which breaks compiling. Worked around this for now by using "<UNKNOWN>" as this

build_files/scons/tools/Blender.py

@@ -277,6 +277,8 @@ def buildinfo(lenv, build_type):
     build_date = time.strftime ("%Y-%m-%d")
     build_time = time.strftime ("%H:%M:%S")
     build_rev = os.popen('svnversion').read()[:-1] # remove \n
+    if build_rev == '': 
+        build_rev = '<UNKNOWN>'
     if lenv['BF_DEBUG']:
         build_type = "Debug"
     else: