configure warnings.

used to work as intended, but got broken some time ago (a quoted empty string
is not an empty string), and got broken some more by the changes to generate
ecpg's preproc.y automatically.  Given all the unprotected uses of $(PERL)
elsewhere, it seems best to make use of the $(missing) script rather than
trying to ensure each such use is protected individually.  Also fix various
bits of documentation that omitted to mention Perl as a requirement for
building from a CVS pull.  Per a complaint from Robert Haas.

When no suitable YACC is configured, supply useful informational messages
to users.  (Same way flex has been handled for a while.)

incarnations (I hope). When an acceptable flex version is not found, print
instructive error messages from both configure and the makefiles, so that
users can continue building anyway.

that now functions as a wrapper around the MakeMaker stuff. It might
even behave sensically when we have separate build dirs. Same for plperl,
which of course still doesn't work very well. Made sure that plperl
respects the choice of --libdir.

Added --with-python to automatically build and install the Python interface.
Works similarly to the Perl5 stuff.

Moved the burden of the distclean targets lower down into the source tree.
Eventually, each make file should have its own.

Added automatic remaking of makefiles and configure. Currently only for the
top-level because of a bug(?) in Autoconf. Use GNU `missing' to work around
missing autoconf and aclocal. Start factoring out macros into their own
config/*.m4 files to increase readability and organization.