two bugs while building back on mingw

[Fred Kiefer]

1. the new backend results in the line
GRAPHIC_LIBS=-ltiff  -lX11
being written to the files config.make and back.make even when not 
configured for any X backend.

2. As the new backend works with subproject these must now be able to 
correctly set set the XXX_ISDLL flags as well. Currently we have this 
correct for applications, libraries and bundles, but not for 
subprojects. Before adding a new workaround I would prefer a general 
solution that would work with all the different project types.
Perhaps it would be sufficient to extract that code into a shared file, 
set one variable, e.g. ALL_USED_LIBS and than include that shared file.

Fred