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
Revert "stdenv: set correct compiler symbol for gcc 4.0"

This reverts commit f4dc5614953b3b90acc28c83262d56587b0616b1.
Revert "default_compiler: recognize gcc-4.0"

This reverts commit bedcbb806bb8a991a1317b4ccde8d16b6974cc46.
Revert "default_cc: locate already returns Pathnames"

This reverts commit 8ec6123d439e8f4d3586bbca6a426b726bd5e9dd.
Revert "stdenv: fix self.cc check"

This reverts commit 0e1d52c0d3945ae0ac69383c7734e425b7fbf8f1.
Recommit ENV changes
ENV.gcc: remove raise

This raise happened in no other ENV methods, and isn't really necessary
since fails_with guards against this method actually being called unless
gcc-4.2 is actually installed.
byobu 5.69

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
stdenv tests: add gcc-4.0 test
notmuch 0.17

update the notmuch formula to version 0.17

Closes #25637.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
libbson 0.4.0

Closes #25639.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
formula: fix whitespace.
ENV: add method to prepend and create a PATH.