updates to the OS X build system :

- XCode project
- Scons :
    scons stopped working for Os X in the last month (dont know when) :

     * the '.' and '..' keywords in CCPPATH
          are not recognized anymore for sconscripts compiling files at
          a sub level
     * when doing a scons clean, the 3 subdirs in build dir are removed
      and scons then fail to recreate them (exten, intern, source)

this commit solve the first problem as a temp workaround
for the latter simply recreate the dirs manually

I will investigate that further when on vacations,  which i should already
 be.
 ;(

all 3 build systems are now working on Os X

intern/ghost/SConscript

@@ -35,6 +35,7 @@ else:
     print "Unknown window system specified."
 
 ghost_env.Append (CPPPATH = ['.',
+                             '../ghost',
                              '../string'])
 
 ghost_env.Append (CPPPATH = user_options_dict['OPENGL_INCLUDE'])