[RFC] Build and install libtac as shared library

Motivation for this is to enable libtac usage for other applications - additionally to pam_tacplus.

Note: this breaks the Debian packaging - if in general the idea is accepted to build libtac as shared library I could provide additional pull request to enable the Debian packaging to build libtac0 and libtac-dev.

Additionally introduce a --enable-pamdir parameter which allows to configure the final pam module location without too much install hooks in automake - by running:

./configure --prefix=/usr --enable-pamdir= /lib/$(DEB_HOST_MULTIARCH)/security

e.g. for Debian packaging. Note: libtool requires that the value for enable-pamdir has no trailing / character. --enable-pamdir=/lib/security/ would fail in the make install target.