Autoconf, Automake and Libtool

We need these now for Xcode-4.3/CLT4X installations.

Also prevent m4 error in installer. And prevent brew doctor complaining if we're Xcode 4.3 or above.

Closes Homebrew/homebrew#10349. Fixes Homebrew/homebrew#10423. Refs Homebrew/homebrew#9179.

Library/Homebrew/cmd/doctor.rb

@@ -620,6 +620,8 @@ def check_git_newline_settings
 end
 
 def check_for_autoconf
+  return if MacOS.xcode_version >= "4.3"
+
   autoconf = `/usr/bin/which autoconf`.chomp
   safe_autoconfs = %w[/usr/bin/autoconf /Developer/usr/bin/autoconf]
   unless autoconf.empty? or safe_autoconfs.include? autoconf

Library/Homebrew/formula_installer.rb

@@ -319,6 +319,8 @@ class FormulaInstaller
   end
 
   def check_m4
+    return if MacOS.xcode_version.to_f >= 4.3
+
     # Check for m4 files
     if Dir[f.share+"aclocal/*.m4"].length > 0 and not in_aclocal_dirlist?
       opoo 'm4 macros were installed to "share/aclocal".'