Check if AC_SYS_LARGEFILE is actually provided

[Sam Liddicott]

It's nice to be able to put

AC_SYS_LARGEFILE

in the configure.in and have largefile support enabled automatically.

Unless of course the builder has an earlier version of autoconf installed.

In which case it would be nice not to have the undefined macros error.

It would be even nice to be able to tell whether or not AC_SYS_LARGEFILE is
defined, and if not, include the definition perhaps that came with GNU tar,
for example.

But it would be a shame to override the built-in system-specicifc
AC_SYS_LARGEFILE with the one from GNU tar on later versions of autoconf.


How should I check whether or not a macro has been defined?

I can't find a way to do it, m4 directives don't work cos the macro gets
expanded first, even with [] and [[]] quoting!

Sam