pathmax: PATH_MAX: use pathconf only when available * lib/pathmax.h (PATH_MAX): Select the pathconf-using definition only if HAVE_PATHCONF is defined. Patch by Sylvain Beucler. * m4/pathmax.m4 (gl_PATHMAX): Check for pathconf. This avoids a link failure in a PSP cross-compilation environment described in http://thread.gmane.org/gmane.comp.lib.gnulib.bugs/17048