Currently `brew install —build-from-source wget` builds all the
dependencies also from source. I can see people wanting to do this when
`HOMEBREW_BUILD_FROM_SOURCE` is set by passing it on the command-line
is mostly just annoying; it means you have to use `—build-bottle` and
deal with the CFLAGS and `post_install` changes if you want to build
from source. Tweak `formula_installer` so this behaviour is more
intuitive.
ClosesHomebrew/homebrew#47889.
Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>