Only track C++ stdlibs for C++ code

After a formula is built, scan all mach-o files for dynamic links
to see if any of them point to a C++ stdlib (libc++ or libstdc++).
If one of them is linked, record that information in the formula's tab.

This replaces the old behaviour where all files were assumed to be C++
code, and stdlibs were always tracked regardless of whether they were
actually linked against.

This also modifies the way that tabs are written - now tabs are written
with the stdlib field null, and values are only written if an stdlib
is detected.

Library/Homebrew/build.rb

@@ -171,7 +171,7 @@ class Build
 
         begin
           f.install
-          Tab.create(f, :libstdcxx, ENV.compiler,
+          Tab.create(f, ENV.compiler,
             Options.coerce(ARGV.options_only)).write
         rescue Exception => e
           if ARGV.debug?

Library/Homebrew/extend/pathname.rb

@@ -390,6 +390,17 @@ class Pathname
     end
   end
 
+  # Returns an array containing all dynamically-linked libraries, based on the
+  # output of otool. This returns the install names, so these are not guaranteed
+  # to be absolute paths.
+  # Returns an empty array both for software that links against no libraries,
+  # and for non-mach objects.
+  def dynamically_linked_libraries
+    `otool -L "#{expand_path}"`.chomp.split("\n")[1..-1].map do |line|
+      line[/\t(.+) \([^(]+\)/, 1]
+    end
+  end
+
   # We redefine these private methods in order to add the /o modifier to
   # the Regexp literals, which forces string interpolation to happen only
   # once instead of each time the method is called. This is fixed in 1.9+.

Library/Homebrew/formula_installer.rb

@@ -349,6 +349,8 @@ class FormulaInstaller
 
     fix_install_names
 
+    record_cxx_stdlib
+
     ohai "Summary" if ARGV.verbose? or show_summary_heading
     unless ENV['HOMEBREW_NO_EMOJI']
       print "\xf0\x9f\x8d\xba  " if MacOS.version >= :lion
@@ -490,6 +492,18 @@ class FormulaInstaller
     @show_summary_heading = true
   end
 
+  def record_cxx_stdlib
+    stdlibs = Keg.new(f.prefix).detect_cxx_stdlibs
+    return if stdlibs.empty?
+
+    tab = Tab.for_formula(f)
+    tab.tabfile.unlink
+    # It's technically possible for the same lib to link to multiple C++ stdlibs,
+    # but very bad news. Right now we don't track this woeful scenario.
+    tab.stdlib = stdlibs.first
+    tab.write
+  end
+
   def clean
     ohai "Cleaning" if ARGV.verbose?
     if f.class.skip_clean_all?

Library/Homebrew/keg_fix_install_names.rb

@@ -42,6 +42,21 @@ class Keg
     end
   end
 
+  # Detects the C++ dynamic libraries in place, scanning the dynamic links
+  # of the files within the keg.
+  # Note that this doesn't attempt to distinguish between libstdc++ versions,
+  # for instance between Apple libstdc++ and GNU libstdc++
+  def detect_cxx_stdlibs
+    results = Set.new
+    mach_o_files.each do |file|
+      dylibs = file.dynamically_linked_libraries
+      results << :libcxx unless dylibs.grep(/libc\+\+.+\.dylib/).empty?
+      results << :libstdcxx unless dylibs.grep(/libstdc\+\+.+\.dylib/).empty?
+    end
+
+    results.to_a
+  end
+
   private
 
   OTOOL_RX = /\t(.*) \(compatibility version (\d+\.)*\d+, current version (\d+\.)*\d+\)/

Library/Homebrew/tab.rb

@@ -10,7 +10,7 @@ require 'utils/json'
 class Tab < OpenStruct
   FILENAME = 'INSTALL_RECEIPT.json'
 
-  def self.create f, stdlib, compiler, args
+  def self.create f, compiler, args
     f.build.args = args
 
     sha = HOMEBREW_REPOSITORY.cd do
@@ -25,8 +25,7 @@ class Tab < OpenStruct
             :tapped_from => f.tap,
             :time => Time.now.to_i, # to_s would be better but Ruby has no from_s function :P
             :HEAD => sha,
-            :compiler => compiler,
-            :stdlib => stdlib
+            :compiler => compiler
   end
 
   def self.from_file path