development_tools: Make GCC always be the default compiler on Linux
development_tools: Make GCC always be the default compiler on Linux

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 linuxbrew/brew#69

Closes Linuxbrew/brew#70.

Signed-off-by: Bob W. Hogg <c772a964fd55352a3510e5d535dd9ccc9ac30168@linux.com>

maxim-belkin/brew,rwhogg/brew,maxim-belkin/brew,rwhogg/brew,maxim-belkin/brew,Linuxbrew/brew,Linuxbrew/brew,hanxue/linuxbrew,rwhogg/brew,hanxue/linuxbrew,Linuxbrew/brew,hanxue/linuxbrew,Linuxbrew/brew