configure: Do not abort when cross-compiling to the native CPU

configure: Do not abort when cross-compiling to the native CPU

Using a compiler with a different host triplet is considered
cross-compiling, even when it is for the same architecture as the
build system.  With such a cross-compiler, it is still valid to
optimize builds with --cpu=host.  Make the condition that aborts in
this case into a warning instead, since a cross-compiler for an
incompatible architecture will fail with -mtune=native anyway.

Signed-off-by: David Michael <fedora.dm0@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>