build-sys: remove .la files after installation

Define KEEP_LA_FILES to keep them.

The hook is repeated because both install-exec-hook and
install-data-hook can install libraries and with parallel make
it's not possible to predict which one will run first.

http://lists.freedesktop.org/archives/systemd-devel/2013-January/008016.html
tl;dr: Libtool .la files are not very useful for linking linux
libraries.