Windows Build system fixes

[Geert Janssens]

All,

Thanks for your feedback on th guile 1.8/gcc 4.5.2 issue.

I have looked into it in more detail and it turned out to be relatively easy 
to make mingw (and hence gcc) more volatile, just like all the other 
dependencies.

I have commited a set of 4 patches:
http://svn.gnucash.org/trac/changeset/20452
http://svn.gnucash.org/trac/changeset/20454
http://svn.gnucash.org/trac/changeset/20455
http://svn.gnucash.org/trac/changeset/20456

The first patch is to make sure that our build scripts always use the mingw 
that is set via MINGW_DIR. This allows to have multiple build directories, 
each using its own mingw while at the same time only one MSYS installation is 
necessary on the system.

Patches 2 and 3 are mainly to fix the automated build setup on our build 
server. They are not relevant for people doing manual builds.

The fourth patch contains some small fixes to the previous three.

I would like to backport these four patches to the 2.4 branch also. The reason 
is that all separate build directories should be aware of the mingw juggling 
or we might hit some subtle errors. Also the patches setup a weekly build of 
the 2.4 branch, and the installers are now uploaded to paths on the webserver 
that make more sense. But this will only work properly on the 2.4 branch if 
it's also patched with the above changes.

I will wait at least until tomorrow before backporting, just to see if 
tonight's build doesn't reveal bugs in my changes.

Geert