Enable 'subdir-objects' automake option

libgd's Makefile.am refers to source files in subdirectories, which
causes warnings with automake 1.14. Enabling the 'subdir-objects'
option silences these.

libgd/Makefile.am:29: warning: source file 'libgd/gd-types-catalog.c' is in a subdirectory,
libgd/Makefile.am:29: but option 'subdir-objects' is disabled
automake: warning: possible forward-incompatibility.
automake: At least a source file is in a subdirectory, but the 'subdir-objects'
automake: automake option hasn't been enabled.  For now, the corresponding output
automake: object file(s) will be placed in the top-level directory.  However,
automake: this behaviour will change in future Automake versions: they will
automake: unconditionally cause object files to be placed in the same subdirectory
automake: of the corresponding sources.
automake: You are advised to start using 'subdir-objects' option throughout your
automake: project, to avoid future incompatibilities.

https://bugzilla.gnome.org/show_bug.cgi?id=732190