build_environment: pass http_proxy env vars through to build env

This allows you to build bottles in an environment that doesn't have
access to the source URLs, or things downloaded during the bottling
phase, without a proxy.

These are the set of environment variables that are already documented
in `man brew`.

Author: gibfahn

Library/Homebrew/build_environment.rb

@@ -49,6 +49,7 @@ def env(*settings)
     CMAKE_PREFIX_PATH CMAKE_INCLUDE_PATH CMAKE_LIBRARY_PATH CMAKE_FRAMEWORK_PATH
     MACOSX_DEPLOYMENT_TARGET PKG_CONFIG_PATH PKG_CONFIG_LIBDIR
     HOMEBREW_DEBUG HOMEBREW_MAKE_JOBS HOMEBREW_VERBOSE
+    all_proxy ftp_proxy http_proxy https_proxy no_proxy
     HOMEBREW_SVN HOMEBREW_GIT
     HOMEBREW_SDKROOT
     MAKE GIT CPP