[C] autoconf fixes.

COMMIT The following, previously posted, patches have been committed. 2005-03-15 Stephen J. Turnbull <stephen(a)xemacs.org&gt; * configure.ac (--with-cflags-optimization): (--with-cflags-debugging): If the user specifies them, use them, and don't make her beg. * configure.ac (Autodetect PNG): PNG & TIFF need -lz in AC_CHECK_LIB. 2005-03-15 Malcolm Purvis <malcolmp(a)xemacs.org&gt; * configure.ac: Fix problems with OpenBSD /bin/sh. -- Malcolm Purvis <malcolmp(a)xemacs.org&gt;