Add support for testing generic OS.

If the environment variable HOMEBREW_TEST_GENERIC_OS is set ensure that
neither Mac nor Linux-specific code is loaded. This allows easier
testing of cross-platform code on OS X and will make it easier to port
Homebrew to platforms other than OS X and Linux.

Library/Homebrew/hardware.rb

@@ -55,8 +55,6 @@ class Hardware
   elsif OS.linux?
     require "os/linux/hardware"
     CPU.extend LinuxCPUs
-  else
-    raise "The system `#{`uname`.chomp}' is not supported."
   end
 
   def self.cores_as_words

Library/Homebrew/os.rb

@@ -1,6 +1,6 @@
 module OS
   def self.mac?
-    /darwin/i === RUBY_PLATFORM
+    /darwin/i === RUBY_PLATFORM && !ENV["HOMEBREW_TEST_GENERIC_OS"]
   end
 
   def self.linux?
@@ -19,9 +19,5 @@ module OS
   elsif OS.linux?
     ISSUES_URL = "https://github.com/Homebrew/linuxbrew/wiki/troubleshooting"
     PATH_OPEN = "xdg-open"
-    # compatibility
-    ::MACOS_FULL_VERSION = ::MACOS_VERSION = "0"
-  else
-    raise "Unknown operating system"
   end
 end

Library/Homebrew/os/mac.rb

@@ -12,6 +12,8 @@ module OS
 
     ::MacOS = self # compatibility
 
+    raise "Loaded OS::Mac on generic OS!" if ENV["HOMEBREW_TEST_GENERIC_OS"]
+
     # This can be compared to numerics, strings, or symbols
     # using the standard Ruby Comparable methods.
     def version

Library/Homebrew/utils.rb

@@ -448,7 +448,9 @@ def exec_editor(*args)
 end
 
 def exec_browser(*args)
-  browser = ENV["HOMEBREW_BROWSER"] || ENV["BROWSER"] || OS::PATH_OPEN
+  browser = ENV["HOMEBREW_BROWSER"] || ENV["BROWSER"]
+  browser ||= OS::PATH_OPEN if defined?(OS::PATH_OPEN)
+  return unless browser
   safe_exec(browser, *args)
 end
 

Library/brew.rb

@@ -141,7 +141,7 @@ rescue RuntimeError, SystemCallError => e
 rescue Exception => e
   Utils::Analytics.report_exception(e)
   onoe e
-  if internal_cmd
+  if internal_cmd && defined?(ISSUES_URL)
     $stderr.puts "#{Tty.white}Please report this bug:"
     $stderr.puts "    #{Tty.em}#{OS::ISSUES_URL}#{Tty.reset}"
   end