AC_TRY_LINK not correct

[Kean Johnston]

A change was made to autoconf sometime between 2.13 and 2.59
that breaks the efficacy of some of the checks in libstdc++,
or anything else for that matter. It has to do with AC_TRY_LINK.
The comment says that the change was to get around the fact
HP-UX CC under certain optimizing conditions (-O3) and the
AIX linker that does somethiong wrong too. Look at the comment
above AC_LANG_FUNC_LINK_TRY in c.m4.

The problem is the new code there does a #undef $1. This is
what is breaking things, and its wrong. It is a perfectly
valid thing for an OS to use #define to implement some function
in terms of another. For example, to implement isinf() in terms
of a call to fpclassify().

The check should probably be extended to try multiple mechanisms
for checking things, and at least one of those mechanisms should
not do that #undef.

Kean