Eliminate MKDIR, srcdir, objdir. Centralize setting of LIBPQDIR.

src/Makefile

src/Makefile.global

src/bin/Makefile

src/bin/psql/Makefile

src/interfaces/libpgtcl/Makefile

src/interfaces/libpq++/Makefile

src/interfaces/libpq++/examples/Makefile

src/test/bench/Makefile

src/test/examples/Makefile

src/test/regress/Makefile

src/tutorial/C-code/Makefile

src/tutorial/Makefile