strndup module and crosscompiling

[Martin Lambers]

Hi!

gnulib/m4/strndup.m4 checks for a deficient strndup() function. When
crosscompiling, it assumes that strndup() is broken if the target is AIX
and that it is ok otherwise. 

This fails when crosscompiling to a target that has no strndup()
function at all, such as MinGW: HAVE_STRNDUP will wrongly be defined to
1.

I don't know how to cleanly integrate a check for the existence of
strndup() into the existing strndup.m4 code, therefore I cannot propose
a patch, sorry.

Martin