* mkfixinc.sh: Check error code from 'cat'. Don't try running

	built fixincl program.
	* mkheaders.in: Use @SHELL@ rather than ${SHELL} to avoid picking
	up csh from user.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@91109 138bc75d-0d04-0410-961f-82ee72b054a4