Log Message:
Add the required glue to make this work again on NetBSD/powerpc.

Pkgsrc changes:
 * Add required macros for accessing ucontext for NetBSD/powerpc
 * Omit files from PLIST.opt which are in PLIST.natdynlink.
   So far macppc doesn't do "natdynlink", tests are failing.
 * Adapt Makefile to features enabled on NetBSD/powerpc.
 * Bump PKGREVISION.

The self-test results are nearly the same as on NetBSD/i386, with one
test failing, difference is one unexpected failure related to native
dynlink which I thought I had not enabled, and which obviously needs
more attention since it tries to reference a Linux linker script.
...
Summary:
  637 tests passed
   13 tests skipped
    1 tests failed
    1 unexpected errors
  652 tests considered

List of failed tests:
    tests/tool-ocamldoc-man/Inline_records.mli

List of unexpected errors:
    tests/lib-dynlink-native
...