Move stdlib tracking postinstall

This moves stdlib tracking after the install completes, which allows
the tracking to have access to the actual stdlib in use.

This unfortunately means that builds can error out *after* a build,
resulting in wasted time; however, it reduces false positives, and the
overall user experience is still likely to be better this way.

Library/Homebrew/build.rb

@@ -141,17 +141,6 @@ class Build
       end
     end
 
-    # TODO Track user-selected stdlibs, such as boost in C++11 mode
-    stdlib = ENV.compiler == :clang ? MacOS.default_cxx_stdlib : :libstdcxx
-    stdlib_in_use = CxxStdlib.new(stdlib, ENV.compiler)
-
-    # This is a bad place for this check, but we don't have access to
-    # compiler selection within the formula installer, only inside the
-    # build instance.
-    # This is also awkward because we don't actually know yet if this package
-    # will link against a C++ stdlib, but we don't want to test after the build.
-    stdlib_in_use.check_dependencies(f, deps)
-
     f.brew do
       if ARGV.flag? '--git'
         system "git init"
@@ -174,7 +163,17 @@ class Build
 
         begin
           f.install
-          Tab.create(f, ENV.compiler,
+
+          stdlibs = Keg.new(f.prefix).detect_cxx_stdlibs
+          # 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.
+          stdlib_in_use = CxxStdlib.new(stdlibs.first, ENV.compiler)
+          # This will raise and fail the build if there's an
+          # incompatibility.
+          stdlib_in_use.check_dependencies(f, deps)
+
+          Tab.create(f, ENV.compiler, stdlibs.first,
             Options.coerce(ARGV.options_only)).write
         rescue Exception => e
           if ARGV.debug?

Library/Homebrew/formula_installer.rb

@@ -386,8 +386,6 @@ class FormulaInstaller
 
     fix_install_names if OS.mac?
 
-    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
@@ -529,18 +527,6 @@ 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_keg f.prefix
-    tab.tabfile.delete if tab.tabfile
-    # 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/tab.rb

@@ -10,7 +10,7 @@ require 'utils/json'
 class Tab < OpenStruct
   FILENAME = 'INSTALL_RECEIPT.json'
 
-  def self.create f, compiler, args
+  def self.create f, compiler, stdlib, args
     f.build.args = args
 
     sha = HOMEBREW_REPOSITORY.cd do
@@ -25,7 +25,8 @@ 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
+            :compiler => compiler,
+            :stdlib => stdlib
   end
 
   def self.from_file path