Fully automate version updating

This one meets or exceeds the following requirements:

 - Version is checked/updated for every build action when in the git repo.
   Does not require the user to re- ./configure to get the correct version.

 - Version is not updated automatically when using exported tarball source.
   Avoids accidentally getting a wrong version from some other git repo in
   a parent directory of the source, and allows setting the correct version
   for distro package exports.

 - Automatic updating can be manually suppressed.
   For developers doing lots of change/rebuild cycles they don't plan to
   release, when they don't want a full rebuild triggered for every commit,
   and again for every change made immediately after a commit.
   The version will still always be updated if they do a `make dist`.

 - Does not require any manual updating of versions in the mainline git
   repo for each release aside from normal tagging.  The version is
   recorded in one file only, that is automatically generated and will
   never need to be committed.

 - Does not require gnu-make features for the autoconf builds.

It does not currently:

 - Keep a checksum of every source file in tarball releases to mangle the
   version if people modify the tarball source.  Responsible people can
   manually update the version easily though in such cases.

The version.mk file is now only used by the VC project files.  Once they
are updated to use the package_version file too, then it can be deleted
from the repository.

celt/celt.c

@@ -50,6 +50,10 @@
 #include "celt_lpc.h"
 #include "vq.h"
 
+#ifndef PACKAGE_VERSION
+#define PACKAGE_VERSION "unknown"
+#endif
+
 
 int resampling_factor(opus_int32 rate)
 {
@@ -193,7 +197,7 @@ const char *opus_strerror(int error)
 
 const char *opus_get_version_string(void)
 {
-    return "libopus " OPUS_VERSION
+    return "libopus " PACKAGE_VERSION
 #ifdef FIXED_POINT
           "-fixed"
 #endif