Empty allow_undefined_flag in libtool.m4 seems to cause undefined symbol errors on Linux

[Ben Roberts]

Hi all,

I've been trying to adapt a set of libraries and programs to use autoconf, 
automake and libtool instead of being manually configured. One of the features 
of this particular package is that there are some symbols that should be 
satisfied at run time, rather than at compile-and-link time.

Now, when I try to build on a Linux box with GNU ld 2.17.50.0.6-9.el5, I get 
errors about undefined symbols.

That version of the linker can be persuaded to ignore undefined symbols using 
the "--unresolved-symbols=ignore-all" flag.

When I consulted libtool.m4, I found that there's a variable in there that is 
supposed to contain that flag, or its equivalent on different systems: namely, 
allow_undefined_flag (with a suffix for the compiler in certain instances).

The problem: in libtool.m4, allow_undefined_flag is left as an empty string on 
Linux systems.

But having just gotten into this whole area, I didn't want to start trying to 
modify libtool.m4 myself, and in any case it has a nasty habit of being 
overwritten when I run "autoreconf --install --force". Besides, I'm not certain 
that modifying libtool.m4 is what I need to do.

Is modifying, and perhaps preparing a patch for, libtool.m4 the best approach? 
An alternative is, no doubt, to write some kind of Linux-specific workaround in 
my configure.ac, but as far as I know libtool aims to spare me that kind of 
thing...

Thanks in advance,

Ben