Makefile.am: add include dir for AC_CONFIG_MACRO_DIR to work

da331647 added a compatiblity macro for
old versions of pkg-config. However, the file in which that macro
resides was not included. From the autoconf docs: "Note that if you use
aclocal from Automake to generate aclocal.m4, you must also set
ACLOCAL_AMFLAGS = -I dir in your top-level Makefile.am.".

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>