AC_CHECK_HEADERS w/o arguments

[Ralf Corsepius]

Hi,

autoconf-2.57 silently accepts using AC_CHECK_HEADERS without arguments
(Yes, I know this is agains autoconf's documentation), eg. accepts
configure.ac's containing something like this

AC_CHECK_HEADERS
AC_PROG_CC

Neither autoconf, m4 nor any tool else complains about this usage of
AC_CHECK_HEADERS.

This results into configure-scripts containing
..
for ac_headers in
do
..

Some shells accept this kind of for-loops, bug some don't (Including my
RH-8.x bash-2.05b-16 didn't complain, while some RH-7.3's bash-2.05 are
reported to abort), rendering this bug very hard to find.

Ralf