Problem reported by Kevin Cernekee in:
http://lists.gnu.org/archive/html/bug-gnulib/2015-02/msg00109.html
* m4/dup2.m4 (gl_FUNC_DUP2): Don't guess no when cross-compiling
for a Linux kernel. That kernel bug was fixed on 2009-05-11, and
there's little need to cross-compile for older kernels nowadays.
* m4/fcntl.m4 (gl_FUNC_FCNTL): When cross-compiling, guess no only
for systems where the bug is known to occur (AIX, Cygwin, Haiku).