[mac] Fix ftexport.sym target in Jamfile.

* Jamfile: Update the directories of the header files scanned for
ftexport.sym.  They were incorrect since the migration of the
header files, on 2015-06-22.  Either inexisting include/cache
(removed on 2006-03-20) is not needed to be listed explicitly.
Now ftmac.h is scanned only in the case of Mac OS & Mac OS X.
2 files changed