build-sys: use 'test -f' instead of AC_CHECK_FILE

AC_CHECK_FILE fails when cross-compiling. It is intended to be used to
check for files that are used at runtime during build time (e.g.
/etc/passwd, /dev/*) [1]. For files which are only used at build time
'test -f' is sufficient.

[1]: https://lists.gnu.org/archive/html/autoconf/2000-10/msg00018.html