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

source/blender/imbuf/SConscript

@@ -34,6 +34,7 @@ source_files = ['intern/allocimbuf.c',
                 'intern/writeimage.c']
 
 imbuf_env.Append (CPPPATH = ['.',
+                             '../imbuf',
                              '../makesdna',
                              '#/intern/guardedalloc',
                              '../blenlib',