pgsql: Separate targets "make docs" and "make install-docs" for the

Log Message:
-----------
Separate targets "make docs" and "make install-docs" for the documentation

It is no longer installed by default, but included in "make world"/"make
install-world". Documentation updated accordingly.

Also, fix vpathsearch function to work when calling make install-docs
without previous make docs.

Modified Files:
--------------
pgsql:
GNUmakefile.in (r1.57 -> r1.58)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/GNUmakefile.in?r1=1.57&r2=1.58)
pgsql/doc/src/sgml:
Makefile (r1.138 -> r1.139)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/Makefile?r1=1.138&r2=1.139)
installation.sgml (r1.343 -> r1.344)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/installation.sgml?r1=1.343&r2=1.344)