pgsql: Add -lm to SHLIB_LINK for contrib/btree_gist.

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
------
master

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

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