davide authored Jun 06, 2019 and Marge Bot committed Jun 10, 2019

Currently the rule for .hi files just triggers (via need) the rule
for the .o file, and .o rule generates both the .o and .hi file.
Likewise for .o-boot and .hi-boot files. This is a bit of an abuse
of Shake, and in fact shake supports rules with multiple output
with the &%> function. This exact use case appears in Neil
Mitchell's paper *Shake Before Building* section 6.3.