* CFLAGS is now properly set when using older gcc.
* All third-party pjproject targets have been removed.  This fixes
  an issue with older libsrtp in some distros.
* Manually removing the source directory now causes a rebuild.
* EXTERNALS_CACHE_DIR is now properly checked.
* Whitespace fixes.

Change-Id: I98fec6847efc5602a9f41cb95096fd660a49fa60