bootstrap: use better defaults for Haiku

These values are patched into that file by Haiku since 2.8.1.
Haiku no longer defines __BEOS__
check for Haiku only with __HAIKU__
Merge topic 'Haiku' into next

3d2e6a0 check for Haiku only with __HAIKU__
7a1b961 Haiku no longer defines __BEOS__
ed96d9a bootstrap: use better defaults for Haiku