Patch from Enrico:

Patch from Enrico:

In src/support/os_cygwin.C path styles are converted (when needed) from
one style to another. I discovered a couple of cases in which an
unnecessary conversion takes place.

The default output path style in external_path is windows because this
function is also used when adding fonts though a native Windows API call.
However, this call is not made in an X11 build, so it is not necessary
that the output path style is windows.

Log:
Small fixes for cygwin.

	* os_cygwin.C (is_windows_path): avoid unnecessary conversion
	when path is relative
	(convert_path_list): avoid unnecessary conversion when path
	list is empty
	(external_path): for an X11 build it is not necessary that
	the output path style is windows

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@14859 a592a061-630c-0410-9148-cb99ea01b6c8