autoconf already adds the "int main() { ...; return 0; }" statements itself,
and so adding them twice chokes stricter compilers.

I think socklen_t was being tested with C++ to improve the warnings,
but as we don't need a C++ compiler, it's not a good idea to use it.

Also, with libtool 2.2 the C++ compiler is not tested by default
anymore, so the macro would simply fail.

On GNU/Linux and FreeBSD the test should work just as fine as it
is. If you have problems on any other operating systems, please report
and we'll see to fix it.

Both attributes.m4 and pthread.m4 are two reusable M4 macro files,
which can eaily be shared between different projects. For this reason
it's better if they are kept identical between branches.

Instead of using directly the sh if statement, use AS_IF, which makes
it possible for autotools to identify conditionally-called macros, and
allows to avoid M4 ifelse macros.

The MIN/MAX macro are quite often used on the source code to find the
minimum or maximum value between two, instead of defining it per-unit,
check if the system provides them include the right header, otherwise
define them during configure run.