CFLAGS, CXXFLAGS and LDFLAGS influence the tests (for example if they include
-L or -I options), so they should be kept from the invocation of configure
to the subsequent reinvocations via config.status.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>