If a file is created by a %.hi rule, and the actual filename isn't
mentioned in the makefiles, then make will treat it as an 'intermediate
file' and delete it when it is finished.

We'd been lucky so far that .hi files weren't actually being built due
to our rules (but rather, as side-effects of the .o rules). However,
when using -dynamic-too to build, we had a rule
    $1/$2/build/%.$$(dyn_osuf): $1/$2/build/%.$$(v_hisuf)
which meant that building a .dyn_o could cause the rule for the
corresponding .hi to be used, and the .hi may then be deleted later on.
This was exacerbated by a bug in GNU make 3.81 which caused make to
enter an infinite loop if running in parallel mode:
    http://lists.gnu.org/archive/html/bug-make/2013-02/msg00020.html

Adding
    .SECONDARY:
would stop make from deleting the intermediate files. However, this
caused make to take a pathologically long time (it appeared to be
live-locked for 2 hours before I killed it) with our build system.

This patch instead creates lines like
    $(eval $(call hi-rule,libraries/base/dist-install/build/Unsafe/Coerce.dyn_hi libraries/base/dist-install/build/Unsafe/Coerce.hi  : %hi: %o  libraries/base/Unsafe/Coerce.hs))
in the .depend files, which results in a rule like
    libraries/base/dist-install/build/Unsafe/Coerce.dyn_hi libraries/base/dist-install/build/Unsafe/Coerce.hi  : %hi: %o  libraries/base/Unsafe/Coerce.hs ;
which, as the files are now all named in the makefiles, means they are
no longer intermediate files so do not get deleted.