configure: use correct cflags in compiler checks

linux-user build on fedora 11 breaks because fallocate
is broken on that system if -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64
are specified, which is what QEMU uses.

We do have a configure check to catch this and disable fallocate,
however, it turns out that default QEMU_CFLAGS/LDFLAGS were assigned in
script *after* all compiler checks: so during checks we were not running
compiler with same flags that we used for build later.

Fix this by moving QEMU_CFLAGS to before compiler checks, and using
comple_prog when checking for fallocate.  This also fixes the fact that
we do some compiler checks while assigning the flags, right below a
comment that says "no cc tests beyond this point".

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>