Makefile cleanup for interface tree. Now essentially with all the

standard targets and behaviour. Replaced Makefile.in's with
Makefile's and declared the respective variables in Makefile.global.

maintainer-clean target now available at top level, although it does
not work in the backend tree yet.

Cleanup pass over Makefile.shlib, renamed some targets and variables.
The shared library symlink tests are now done by make, not the shell.

ecpg: Remove one warning in sloppy flex output.

PL/Perl and Perl interface: the MakeMaker documentation is confusing,
the realclean target *does* "delete derived files", but it also
uninstalls them. Don't use that.

The submake targets in the various bin directories that update libpq
should `make all', not `make libpq.a'. That is a) unportable, and
b) doesn't build the shared library.