Define a fallback version string.

The new opus_get_version_string() call just returns a static string
defined by OPUS_VERSION, which is passed in from the build system
through config.h (or a custom compile line). Provide a fallback to
"unknown" if the build system fails to actually provide that definition.

This restores compilation with Makefile.draft.

In general, this means there will be builds out there with non-specific
version strings, since ports won't be forced to update the string. While
that's unfortunate, I think it's more valuable that the library be simple
to build.

src/opus.c

@@ -31,6 +31,10 @@
 
 #include "opus.h"
 
+#ifndef OPUS_VERSION
+#define OPUS_VERSION "Opus version unknown"
+#endif
+
 const char *opus_strerror(int error)
 {
    static const char *error_strings[8] = {