Gamin was using its own, debugging, versions of the g_list glib functions.
They were written to find problems in GList usage, but were causing gamin
to segfault on 4.X. Disable debugging by default, and only use the debugging
versions of g_list_* if debugging is enabled.

Additionally, add <string.h> to gam_api.c which would certainly fix problems
on 4.X alpha.

Reported by: Stefan Thurner <thurners@nicsys.de>