add MSYS_NO_PATHCONV=1 to prevent MSYS auto-magic

add MSYS_NO_PATHCONV=1 to prevent MSYS auto-magic

git-bash/MSYS tries to resolve anything that looks like a path (i.e.
anything starting with a /) - if the "target" doesn't exists, it just
prepends the MSYS installation dir and breaks all kind of stuff.
You could escape those with double-slash at the beginning, but that
would mean lots of noise/unnecessary changes in pretty much every
makefile, so just getting rid of that behavior is much better.

ant and some externals however rely on MSYS path-conversion, so unset
MSYS_NO_PATHCONV for those. Ant does its own checks whether it
runs under cygwin or msys, and easier to please externals using the
autoconversion than to add patches to force them into compliance…

Change-Id: Ie80964e40c53ace80adb4707aae0db63ebc4ee4c
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/166330
Tested-by: Jenkins
Reviewed-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>