development_tools: Make GCC always be the default compiler on Linux

[rwhogg]

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For systems like Ubuntu that use hardening-wrapper as the default GCC, DevelopmentTools.default_compiler will assume Clang is the default, causing test failures.

Fix test:
TabTests#test_defaults [$PREFIX/Library/Homebrew/test/test_tab.rb:35]

Closes #69

[rwhogg]

An alternative to this might be else (OS.mac? ? :clang : :gcc).

Part of me prefers that over special-casing in hardening-wrapper, but I'm hesitant to make such a big change when this fixes it.

[sjackman]

I'm thinking that default_compiler should be on Linux:

def default_compiler
  return :gcc unless OS.mac?
  …

For historical reasons, :gcc is an alias for :gcc_4_2 and means GCC 4.2 to Homebrew. It should probably be avoided on Linuxbrew. Does :gnu work? I'm not sure that it does. That's what used in compilers.rb find_compiler, but I don't believe it has any meaning outside of that function. :"gcc-5" could work, but it is not correct for the host's compiler, e.g. :"gcc-4.8".

Until we fix it correctly, I'm okay with the above hack, which ignores the value of /usr/bin/cc.

[rwhogg]

The Circle CI failure is a configuration problem, so merging anyways.

https://circleci.com/gh/Linuxbrew/brew/14

==> brew pull --clean --tap= https://github.com/Linuxbrew/brew/pull/70
==> FAILED
Error: Invalid tap name

[sjackman]

I introduced that --tap= bug, and haven't had a chance to fix it. Should be an easy fix.
https://github.com/Linuxbrew/brew/blob/master/Library/Homebrew/dev-cmd/test-bot.rb#L867