ntpc.so is unable to resolve libpython2.7.1.0 on NetBSD

Following up on a report by Hal on the mailing list: https://lists.ntpsec.org/pipermail/devel/2016-December/002958.html

-bash-4.3$ ldd /usr/local/lib/python2.7/site-packages/ntp/ntpc.so
/usr/local/lib/python2.7/site-packages/ntp/ntpc.so:
        -lpython2.7.1.0 => not found
        -lutil.7 => /usr/lib/libutil.so.7
        -lgcc_s.1 => /lib/libgcc_s.so.1
        -lc.12 => /usr/lib/libc.so.12
        -lm.0 => /usr/lib/libm.so.0
        -lrt.1 => /usr/lib/librt.so.1
        -lpthread.1 => /usr/lib/libpthread.so.1

I'm seeing this on both NetBSD 6 and NetBSD 7, with both python27 and python35. Looks like waf is not passing -R/usr/pkg/lib to the compiler/linker for some reason.

cc: @hal.murray