development_tools: add install instruction helpers.

Library/Homebrew/development_tools.rb

@@ -19,6 +19,12 @@ class DevelopmentTools
       which("clang") || which("gcc")
     end
 
+    def installation_instructions
+      "Install Clang or brew install gcc"
+    end
+    alias_method :custom_installation_instructions,
+                 :installation_instructions
+
     def default_cc
       cc = DevelopmentTools.locate "cc"
       cc.realpath.basename.to_s rescue nil

Library/Homebrew/extend/os/mac/development_tools.rb

@@ -22,6 +22,44 @@ class DevelopmentTools
       MacOS::Xcode.installed? || MacOS::CLT.installed?
     end
 
+    def installation_instructions
+      if MacOS.version >= "10.9"
+      <<-EOS.undent
+        Install the Command Line Tools:
+          xcode-select --install
+      EOS
+      elsif MacOS.version == "10.8" || MacOS.version == "10.7"
+        <<-EOS.undent
+          Install the Command Line Tools from
+            https://developer.apple.com/downloads/
+          or via Xcode's preferences.
+        EOS
+      else
+        <<-EOS.undent
+          Install Xcode from
+            https://developer.apple.com/xcode/downloads/
+        EOS
+      end
+    end
+
+    def custom_installation_instructions
+      if MacOS.version > :tiger
+        <<-EOS.undent
+          Install GNU's GCC
+            brew install gcc
+        EOS
+      else
+        # Tiger doesn't ship with apple-gcc42, and this is required to build
+        # some software that doesn't build properly with FSF GCC.
+        <<-EOS.undent
+          Install Apple's GCC
+            brew install apple-gcc42
+          or GNU's GCC
+            brew install gcc
+        EOS
+      end
+    end
+
     def default_compiler
       case default_cc
       # if GCC 4.2 is installed, e.g. via Tigerbrew, prefer it