build: Make autoreconf honour ACLOCAL_FLAGS

build: Make autoreconf honour ACLOCAL_FLAGS

When running autoreconf, it's possible to give flags to the underlying
aclocal by declaring a ACLOCAL_AMFLAGS variable in the top level
Makefile.am.

Putting ${ACLOCAL_FLAGS} there allows the user to set an environment
variable up before running autogen.sh and pull in the right directories
to look for m4 macros, say an up-to-date version of the xorg-util macros.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Cc: Daniel Stone <daniel@fooishbar.org>