Autodetect lua5.1 (not sure how to detect lua-5.1 or just lua)

SConscript

@@ -77,7 +77,10 @@ except:
             env.Append(CPPPATH=GetOption("sdl-dir"))
 
 #Find correct lua include dir
-if(GetOption("lua-dir")):
+try:
+    env.ParseConfig('pkg-config --cflags lua5.1')
+except:
+    if(GetOption("lua-dir")):
         if not conf.CheckCHeader(GetOption("lua-dir") + '/lua.h'):
             print "lua5.1 headers not found or not installed"
             raise SystemExit(1)