Open Build Service RPMs for mingw32-dbus-1 hard-code all the
directories to make everything explicit, notably:

--prefix=/usr/i686-w64-mingw32/sys-root/mingw
--exec-prefix=/usr/i686-w64-mingw32/sys-root/mingw
...
--libdir=/usr/i686-w64-mingw32/sys-root/mingw/lib

Previously we didn't accept this as relocatable, but actually it's
fine: ${prefix} is still equivalent to ${libdir}/pkgconfig/../..,
so our relocation setup can work. Accept the result of expanding
"${prefix}" as an acceptable value for --exec-prefix, and accept the
results of expanding "${exec_prefix}/lib" etc. as acceptable values
for --libdir.

Note the use of single vs. double quotes here. A case statement that
matches '${prefix}' tests for the literal string «${prefix}»,
whereas a case that matches "${prefix}" tests for the string that is
the value of the variable named «prefix» that is set by the
--prefix command-line argument.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=107662