Convert cvsignore to gitignore, and add .gitignore for build targets.

.gitignore

+# Global excludes across all subdirectories
+*.o
+*.so
+*.so.[0-9]
+*.so.[0-9].[0-9]
+*.sl
+*.sl.[0-9]
+*.sl.[0-9].[0-9]
+*.dylib
+*.dll
+*.a
+*.mo
+objfiles.txt
+.deps/
+
+# Local excludes in root directory
+/GNUmakefile
+/config.log
+/config.status

contrib/adminpack/.gitignore

+/adminpack.sql

contrib/btree_gin/.gitignore

+/btree_gin.sql

contrib/btree_gist/.gitignore

+/btree_gist.sql

contrib/chkpass/.gitignore

+/chkpass.sql

contrib/citext/.gitignore

+/citext.sql

contrib/cube/.cvsignore

-cubeparse.c
-cubescan.c

contrib/cube/.gitignore

+/cubeparse.c
+/cubescan.c
+/cube.sql

contrib/dblink/.gitignore

+/dblink.sql

contrib/dict_int/.gitignore

+/dict_int.sql

contrib/dict_xsyn/.gitignore

+/dict_xsyn.sql

contrib/earthdistance/.gitignore

+/earthdistance.sql

contrib/fuzzystrmatch/.gitignore

+/fuzzystrmatch.sql

contrib/hstore/.gitignore

+/hstore.sql

contrib/intarray/.gitignore

+/_int.sql

contrib/isn/.gitignore

+/isn.sql

contrib/lo/.gitignore

+/lo.sql

contrib/ltree/.gitignore

+/ltree.sql

contrib/oid2name/.gitignore

+/oid2name

contrib/pageinspect/.gitignore

+/pageinspect.sql