dependency: don't try and expand yourself.

This can happen on e.g. fossil where the dependency HEAD requires having
an install of fossil in order to check it out. Bit of a silly edge case
but this handles it.

Fixes #25605
README: add Gittip link.

Closes #20482.
tap: never throw AlreadyTappedError.

Just make it a warning instead.

Closes #25511.
Closes #25617.
install, update: AlreadyTappedError never raised.
exceptions: remove unused AlreadyTappedError.
wry 1.7.1

Closes #25123.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
stdenv: fix self.cc check

Since ENV.cc/cxx are accessors to ENV, any values being passed through
are coerced into strings, which means that a nil `ENV.cc` is actually
an empty string - which isn't considered to be false in Ruby.

Fixes mistydemeo/tigerbrew#161.
Fixes mistydemeo/tigerbrew#135.
default_cc: locate already returns Pathnames
default_compiler: recognize gcc-4.0
stdenv: set correct compiler symbol for gcc 4.0
mitmproxy: switch to pillow; fixes #25522
minizip: needs newer autotools

Fixes mistydemeo/tigerbrew#164.
tor: needs newer openssl on < Snow Leopard

Fixes mistydemeo/tigerbrew#163.
tor: fix conditionals