build: avoid link failure when strerror is replaced

build: avoid link failure when strerror is replaced

When building the new make-prime-list program on a system for which
strerror is defined to rpl_strerror, we'd get a link failure.
The problem is that we're including <config.h> for some definitions,
but do not want the rpl_ ones, since this particular program must
not be linked against gnulib (aka libcoreutils.a).  This did not
arise on Fedora 17 or 18, but did on Debian wheezy/sid.
* src/make-prime-list.c (strerror): #undef.
Build failure introduced by commit v8.19-152-gcf67e4c.