This ensures that the root Makefile runs only a single target at a time.
SUBMAKE will still honor requested parallelism, so 'make -j8' will build
one directory at a time but allow 8 jobs at once when building a sub
directory.

This will fix some display glitches related to rebuild of XML
documentation.  It will also prevent some edge case errors where
bundled pjproject needs to be rebuild before other parts of Asterisk.

Change-Id: I4f2ec6fbbec1ada0ccb1109a28ea303524239b1e