pgbouncer - pgbouncer: Solaris build fixes.

Log Message:
-----------
Solaris build fixes.

- slab: Include <sys/param.h> after system.h
- define _XPG4_2 and __EXTENSIONS__
- disable -Wextra if -Wno-missing-field-initializers cannot be given
- always use $(INSTALL)

Reported & Tested by Hubert Depesz Lubaczewski

Modified Files:
--------------
    pgbouncer:
        configure.ac (r1.64 -> r1.65)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbouncer/pgbouncer/configure.ac?r1=1.64&r2=1.65)
    pgbouncer/doc:
        Makefile (r1.10 -> r1.11)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbouncer/pgbouncer/doc/Makefile?r1=1.10&r2=1.11)
    pgbouncer/include:
        system.h (r1.21 -> r1.22)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbouncer/pgbouncer/include/system.h?r1=1.21&r2=1.22)
    pgbouncer/src:
        slab.c (r1.10 -> r1.11)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbouncer/pgbouncer/src/slab.c?r1=1.10&r2=1.11)