Fix Makefile.am compatibility with automake 1.11.2

Automake 1.11.2 made the following change:

* Long-standing bugs:
  - Automake now warns about more primary/directory invalid combinations,
    such as "doc_LIBRARIES" or "pkglib_PROGRAMS".

Unfortunately, this breaks our Makefile.am (issue 216) exactly because
we were relying on pkglib_SCRIPTS.

This patch works around this by adding a new myexeclibdir variable
(exec so that it is intalled at `install-exec` time, the same as the
pkglibdir), and switches to that.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>