Fix man page detection when cross compiling.

AC_CHECK_FILE(S) is meant for checks of files on the native system and does
not work when cross compiling (see [lib/autoconf/general.m4](http://git.savannah.gnu.org/gitweb/?p=autoconf.git;a=blob;f=lib/autoconf/general.m4;h=59d204f6a3c7a3b24b6af796e8ec179a4f6c93e0;hb=HEAD#l2832)).