Numbering files according to rules causes confusion, because
CUSTOM_COMMAND3.stamp from a previous build might represent
completely different targets after Makefile.ninja is regenerated.
As a result, the new targets are not rebuilt and compilation
fails.
Use the targets to build a SHA1 hash; the chances for collision
are one in 2^24 even with a 12-character prefix of the hash.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
