Updates the configure check to match the one in the Opus
repository.

AC_INIT requires that the version string passed in be 'static'
so we can't use the output of a shell invocation. However, it
can be computed by an m4 shell invocation. This is what autoconf
itself does to embed its git version number in PACKAGE_VERSION.

Doing this necessitates the 'echo -n' in git-version.sh. Make
is smart enough to strip the trailing newline, but AC_INIT is
not. We must also add -f to autoreconf. Otherwise, autogen.sh
will only update configure when configure.ac changes, even if
the output of git-version.sh has changed.

This is still not ideal, since it runs at autogen.sh time, not
at 'make' time, which is what we want. This seems to be a limitation
of an upcoming automake release, which removes the old style
with PACKAGE and VERSION set by shell variables.

This matches what the opus package does, and is consistent with
what the pkg-config files were already doing.

This almost, but doesn't quite, copy the layout the opus
configure script uses.

Diego Elio Pettenò authored Sep 29, 2012 and Timothy B. Terriberry committed Sep 29, 2012

Includes
- A make debug target that disables optimizations and enables
   assertions,
- Proper ./configure switches for the optional features,
- A configuration summary,
- libtool versioning information,
- Visibility and warning flags,
- API documentation, and
- Support for out-of-tree builds.
Signed-off-by: Diego Elio Pettenò <flameeyes@flameeyes.eu>