Update mono config generation

Mono PR mono/mono#1064 changes how the paths for libMonoPosixHelper and
libgdiplus stored in the configuration file are computed. This means
that no fixing is needed anymore for libMonoPosixHelper, because the
intallation prefix is automatically added. Instead libgdiplus is
assumed to be in the default library load path unless another path is
specified at configure-time through the --with-libgdiplus option.
You're right, libgdiplus will not be a dylib on non-darwin systems. On the other hand the appropriate --with-libgdiplus=/path/to/libgdiplus.so would be needed on other systems, too.
Should we assume that bockbuild is darwin-only or should I fix the PR so that it also sets the .so path if the system does not uses dylibs?