--define-prefix is broken in distros that install in multi-arch directories

In Debian and Ubuntu the multiarch filesystem layout looks as follows:

/usr/lib/x86_64-linux-gnu/pkgconfig/ /usr/lib/i386-linux-gnu/pkgconfig/

Which means when --define-prefix skips to the grand-parent directory of pkgconfig, it ends up in /usr/lib rather than /usr as it's supposed to.

This causes for example the include path to be printed as: /usr/lib/include which is wrong and breaks applications.

EG:

$ pkg-config --define-prefix --cflags blkid
-I/usr/lib/include/blkid

This is obviously wrong and causes build failures, as there's no such directory.

An "hacky" fix was proposed in !4 but never went anywhere. It would be great if this bug could be looked at.

Thanks!