The path to WPrefs has been hardcoded in many of the menu files to
/usr/local/GNUstep/Applications/WPrefs.app/WPrefs, which would only actually
work if the user ran something like:
./configure --with-gnustepdir=/usr/local/GNUstep
during build.
Instead, we add a .in extension to all menu files with this issue and use sed
to use the actual WPrefs path (given by the wprefs_bindir output variable) and
generate a new menu file.
A very similar idea is already used to generate the WMState file, which sets
WPrefs as the command for the Window Maker logo tile in the dock.
This patch fixes Debian bug #851737:
https://bugs.debian.org/851737
