Add -lm to SHLIB_LINK for contrib/btree_gist.

Now that btree_gist contains a reference to isinf(), this is necessary
at least on some platforms.  Per buildfarm.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/8c4164540b30434845be02f84973663bfeb231ab

Modified Files
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contrib/btree_gist/Makefile |   18 +++++++++++-------
1 files changed, 11 insertions(+), 7 deletions(-)

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Next:	From: Tom Lane	Date: 2011-03-03 07:08:18
	Subject: Re: [COMMITTERS] pgsql: Add KNNGIST support to contrib/btree_gist.
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	Subject: pgsql: Mark operator implementation functions as such in theircomments