configure: fix non-newlib cross-compiling

Commit 529046c3 "libcflat: add PRI(dux)32 format types" applies a
detection trick that requires native compiling or cross-compiling
with newlib, an embedded system C standard library. We can avoid
the new dependency with a different trick.

Cc: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>