CONFIGURE: Ask for unity >= 3.8.1
Previously we asked for unity == 3.8.1, which broke things everywhere else
CONFIGURE: Use pkgconfig to get CFLAGS for Unity if not supplied

This is not necessary if we got --with-libunity-prefix= (line
1447).
That's not always the case though.
Merge pull request #511 from tobiatesan/unity

CONFIGURE: Fix support for libunity in configure