Port to hosts whose 'sort' and 'tail' implementations treat operands with leading '+' as file names...

	Port to hosts whose 'sort' and 'tail' implementations
	treat operands with leading '+' as file names, as POSIX
	has required since 2001.  However, make sure the code still
	works on pre-POSIX hosts.
	* ltmain.sh: Don't assume "sort +2" is equivalent to
	"sort -k 3", since POSIX 1003.1-2001 no longer requires this.

contrib:
	* compare_tests: Don't assume "sort +2" is equivalent to
	"sort -k 3", since POSIX 1003.1-2001 no longer requires this.
gcc:
	* Makefile.in (slowcompare): Port to POSIX 1003.1-2001,
	which says you should use "tail -c +N" rather than "tail +Nc".
	Fix a bug: the old code incorrectly skipped 15 bytes, not 16.

From-SVN: r115234