Ensure the module is built using the specified mapserver library

This fixes issues with the module potentially linking against the
wrong mapserver library on systems with multiple mapserver installs.
Fix segfault caused by mapfile comments in `Map.FromString`

Mapserver `msLoadMapFromString` (currently at 6.3-dev) fails to parse
mapfile strings with comments and produces an empty error which was
not properly managed in the module.
Clean up the IO handlers after a mapserver request

This fixes a memory leak.
Created version 0.1.1