Build System: Disable parallel make in the root Makefile.

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

Makefile

@@ -1111,3 +1111,6 @@ check-alembic: makeopts
 
 FORCE:
 
+# This only stops targets within the root Makefile from building in parallel.
+.NOTPARALLEL:
+