pgsql: When cross-compiling, allow and require an external zic program

Log Message:
-----------
When cross-compiling, allow and require an external zic program to be used
when --with-system-tzdata is not used.

initial patch by Richard Evans

Modified Files:
--------------
    pgsql:
        configure (r1.619 -> r1.620)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/configure?r1=1.619&r2=1.620)
        configure.in (r1.578 -> r1.579)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/configure.in?r1=1.578&r2=1.579)
    pgsql/src:
        Makefile.global.in (r1.250 -> r1.251)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/Makefile.global.in?r1=1.250&r2=1.251)
    pgsql/src/timezone:
        Makefile (r1.29 -> r1.30)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/timezone/Makefile?r1=1.29&r2=1.30)