Make it clear that we've done the right thing and this message is just
for informational purposes.

The post-install audit methods are not run for keg-only installs, so we
don't need to repeat the conditional here.

This is a follow-up to bbf9f7460fc8e6e4c63f5bbe8630b11ca53f3710.

If we read from the pipe before calling Process.wait, we can avoid
spawning a thread to do the read while continuing to avoid the deadlock
that the thread was introduced to solve.

Long term we should probably rename the installer's "f" attribute to
"formula".

None of these subclasses share any behavior other than what is inherited
from RuntimeError, so we can just get rid of the superclass.

This reverts commit c8d3b39165bb11799d7849ee13a3559ad0bd63f4.

Fixes Homebrew/homebrew#28754.
Fixes Homebrew/homebrew#29846.
Fixes Homebrew/homebrew#30920.

BuildOptions is now immutable (finally).

This reverts commit 8d2ef974a3a87bf4207f71ccb8a7b4776e16a016.

build.without? seemingly always returns true for requirements that are
neither recommended nor optional so check these before deciding not to
install a default formula.

Closes Homebrew/homebrew#31511.

Use the build options instead of whether the requirement is optional to
work out if it is needed. This handles the case of an optional
requirement becoming needed because it's a dependency for another
formula.

Closes Homebrew/homebrew#31476.

This reverts commit 435092ae0521e79d8ea21c8d4708031e3b4b17a3.

(Commit message written by Mike McQuaid)

With the change in a0a93f1b3b7b2be9b8a319be91086ffe220f8e32
unfortunately requirements with default formulae no longer could be
resolved when the default formulae were not installed. This commit
fixes this problem.

Closes Homebrew/homebrew#31476.
Closes Homebrew/homebrew#31444.
Closes Homebrew/homebrew#30901.
Closes Homebrew/homebrew#31471.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

If the build process tries to write more bytes than will fit in the pipe
buffer, it will block until the parent process does a read. However, the
parent process will only do a read after the child process has exited or
died, leading to a deadlock. Fix this by doing the read in a separate
thread.