pgsql: Fix new timezone cross-compile rule to avoid a bug in gmake

Log Message:
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Fix new timezone cross-compile rule to avoid a bug in gmake 3.78.1;
document change.

Modified Files:
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pgsql/src/timezone:
Makefile (r1.30 -> r1.31)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/timezone/Makefile?r1=1.30&r2=1.31)