build: Update ACLOCAL_AMFLAGS to install third-party macros
build: Update ACLOCAL_AMFLAGS to install third-party macros

So that they are carried around in the distributed tarball, and there
are no dependencies on things like autoconf-archive.

https://wiki.gnome.org/Projects/GnomeCommon/Migration

GNOME/libgdata,pwithnall/libgdata,pwithnall/libgdata