GNU make provides a default definition for this variable.

I've no idea what's special about mingw32-ar vs native ar,
but it seems to be required.

This is pretty preliminary for now.
Seeking performance isn't great, and there's no caching.
I'm sure there's also lots of bugs.

If a source file moves (as in the previous commit), any stale
dependencies will block execution of 'make' because they
define a dependency relationship to an non-existant file.

The obvious thing to do is 'make clean' to get rid of the
dependency files, but of course that fails the same way.
To support this recovery, skip the include directive if
the GNU make variable MAKECMDGOALS is 'clean', exempting
this target from dependency checks.

Based on an example in the GNU make manual.

The actual problem was that opus-uninstalled.pc was forcing static
 linking, but not including all the relevant libraries.

This is needed to pick up -lm for libopus when it requires it.

It's not actually used by the examples anymore.

Needed to build on my Fedora 16 machine.

Seeking and playback work, at least as far as the examples test
 them.
There are probably still lots of bugs.