This simplifies the docs and makes it easier to cut/paste command lines.

modules are built.  Foremost, it creates a solid distinction between these two
types of targets based on what had already been implemented and duplicated in
ad hoc ways before.  Specifically,

- Dynamically loadable modules no longer get a soname.  The numbers previously
set in the makefiles were dummy numbers anyway, and the presence of a soname
upset a few packaging tools, so it is nicer not to have one.

- The cumbersome detour taken on installation (build a libfoo.so.0.0.0 and
then override the rule to install foo.so instead) is removed.

- Lots of duplicated code simplified.

installations whose pg_config program does not appear first in the PATH.
Per gripe from Eddie Stanley and subsequent discussions with Fabien Coelho
and others.

to avoid useless multiple executions of pg_config.

tutorial to be used without necessarily having a configured source tree.

Eliminate the mysterious games that the Cygwin build plays with the linker
flag variables.  DLLLIBS is gone, use SHLIB_LINK like everyone else.
Detect cygipc in configure, after the linker flags are set up, otherwise
configure might not work at all.

Make sure everything is covered by make clean.

Fix the build of the new conversion procedure modules.

Add new DLLIMPORT markers where required.

Finally, the compiler complains if we use an explicit
-I/usr/local/include, so don't do that.  Curiously, -L/usr/local/lib is
still necessary.