build-sys: Don't add libkmod subdirectory to include path

Like 05437b8 (Makefile.am: Don't add abc subdirectory to include path)
in libabc by Josh Triplett.
build-sys: Don't define LIBEXECDIR

Like b2fd181 (Makefile.am: Don't define LIBEXECDIR) in libabc by Josh
Triplett.
build-sys: set -e separately, rather than putting -e in the shebang line

Like a008d76 (autogen.sh: set -e separately, rather than putting -e in
the shebang line) in libabc by Josh Triplett.
Remove FSF mailing address

It has changed in the past, and these days, anyone can get a copy of the
LGPL via the web rather than by post.

Like 657a122 (Remove FSF mailing address) in libabc by Josh Tripplet,
but let the FSF website in which the license can be found.