pgsql: Add GIN support for pg_trgm.

Log Message:
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Add GIN support for pg_trgm. From Guillaume Smet <guillaume(dot)smet(at)gmail(dot)com>
with minor editorization by me.

Modified Files:
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pgsql/contrib/pg_trgm:
Makefile (r1.6 -> r1.7)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_trgm/Makefile.diff?r1=1.6&r2=1.7)
README.pg_trgm (r1.2 -> r1.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_trgm/README.pg_trgm.diff?r1=1.2&r2=1.3)
pg_trgm.sql.in (r1.2 -> r1.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_trgm/pg_trgm.sql.in.diff?r1=1.2&r2=1.3)
trgm.h (r1.6 -> r1.7)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_trgm/trgm.h.diff?r1=1.6&r2=1.7)
uninstall_pg_trgm.sql (r1.2 -> r1.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_trgm/uninstall_pg_trgm.sql.diff?r1=1.2&r2=1.3)
pgsql/contrib/pg_trgm/expected:
pg_trgm.out (r1.1 -> r1.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_trgm/expected/pg_trgm.out.diff?r1=1.1&r2=1.2)
pgsql/contrib/pg_trgm/sql:
pg_trgm.sql (r1.1 -> r1.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_trgm/sql/pg_trgm.sql.diff?r1=1.1&r2=1.2)

Added Files:
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pgsql/contrib/pg_trgm:
trgm_gin.c (r1.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_trgm/trgm_gin.c?rev=1.1&content-type=text/x-cvsweb-markup)