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.

contrib/btree_gist/Makefile

 # contrib/btree_gist/Makefile
 
-MODULE_big  = btree_gist
+MODULE_big = btree_gist
 
-OBJS        = btree_gist.o btree_utils_num.o btree_utils_var.o btree_int2.o btree_int4.o btree_int8.o \
-              btree_float4.o btree_float8.o btree_cash.o btree_oid.o btree_ts.o btree_time.o \
-              btree_date.o btree_interval.o btree_macaddr.o btree_inet.o btree_text.o \
-              btree_bytea.o btree_bit.o btree_numeric.o
+OBJS =  btree_gist.o btree_utils_num.o btree_utils_var.o btree_int2.o \
+        btree_int4.o btree_int8.o btree_float4.o btree_float8.o btree_cash.o \
+        btree_oid.o btree_ts.o btree_time.o btree_date.o btree_interval.o \
+        btree_macaddr.o btree_inet.o btree_text.o btree_bytea.o btree_bit.o \
+        btree_numeric.o
 
 EXTENSION = btree_gist
 DATA = btree_gist--1.0.sql btree_gist--unpackaged--1.0.sql
 
-REGRESS     = init int2 int4 int8 float4 float8 cash oid timestamp timestamptz time timetz \
-              date interval macaddr inet cidr text varchar char bytea bit varbit numeric not_equal
+REGRESS = init int2 int4 int8 float4 float8 cash oid timestamp timestamptz \
+        time timetz date interval macaddr inet cidr text varchar char bytea \
+        bit varbit numeric not_equal
+
+SHLIB_LINK += $(filter -lm, $(LIBS))
 
 ifdef USE_PGXS
 PG_CONFIG = pg_config