Link the include dir into out-of-tree build dirs

Mostly this is for people building other things using an uninstalled
opus tree so that the opus-uninstalled.pc will return a working -I
with --cflags when PKG_CONFIG_PATH is set to point to the build dir.

The version of AC_OUTPUT that takes parameters has long been
deprecated now, and replaced by AC_CONFIG_* macros, so pass them
with AC_CONFIG_FILES, since we're now using AC_CONFIG_COMMANDS
to do the above.