On Windows, improve detection of the TeX engine by avoiding spaces in

On Windows, improve detection of the TeX engine by avoiding spaces in
filenames passed to latex (through the short form of the path) and also
by taking into account that a short form may not always be available.
Such detection may still fail if the file system does not support short
names and the TeX engine doesn't allow spaces in filenames, but this is
a highly improbable condition, hopefully.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/branches/BRANCH_2_0_X@38921 a592a061-630c-0410-9148-cb99ea01b6c8