gdb: Update to 7.8 and include Guile support.

GDB 7.8 adds Guile scripting support.  This is now enabled by default as
well.  Checking for guile also adds a pkg-config dependency.

It seems Python is explicitly excluded from cross building.  I'm not
sure if Guile should also be excluded.
disable guile by default and add attribute for gdb-with-guile