Don't use a global variable for avoiding removal of the system temp dir

Don't use a global variable for avoiding removal of the system temp dir

	* src/support/package.h
	(Package::system_temp_dir): new, return the system temp dir.

	* src/support/package.C.in
	(Package::Package): record the system temp dir.

	* src/lyx_main.C
	(LyX::prepareExit): don't remove the temporary directory if it
	is the same as the system temp dir, meaning that the lyx_tmpdir
	has not yet been created.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@16318 a592a061-630c-0410-9148-cb99ea01b6c8