Cross compiling with pkg-config

[Keith Packard]

Yeah, yeah, I know there are some hacks in pkg-config for cross compiling,
but they're not sufficient.  The problem is that prefix needs to point at
the target installation directory so that file names in the package will
work correctly while the package is running.  However, prefix must also
point at the files on the build host so that the header files and .la files
will be found while compiling.

Essentially, we need to prepend whatever DESTDIR was used to install the 
development pieces of the packages.  A simple kludge is to add a 
environment variable which does precisely this -- edits the prefix entry.

Seem reasonable?

-keith