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/memutil/SConscript

@@ -6,6 +6,6 @@ memutil_env = library_env.Copy ()
 
 source_files = ['intern/MEM_RefCountedC-Api.cpp']
 
-memutil_env.Append (CPPPATH = ['.'])
+memutil_env.Append (CPPPATH = ['.','../memutil'])
 
 memutil_env.Library (target='#'+user_options_dict['BUILD_DIR']+'/lib/blender_MEM', source=source_files)