C++ stdlibs: issue warning, don't fail the build

Library/Homebrew/build.rb

@@ -168,13 +168,15 @@ class Build
           # want to record the stdlib for something that installs no
           # dylibs.
           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.
+          # This currently only tracks a single C++ stdlib per dep,
+          # though it's possible for different libs/executables in
+          # a given formula to link to different ones.
           stdlib_in_use = CxxStdlib.new(stdlibs.first, ENV.compiler)
-          # This will raise and fail the build if there's an
-          # incompatibility.
+          begin
             stdlib_in_use.check_dependencies(f, deps)
+          rescue IncompatibleCxxStdlibs => e
+            opoo e.message
+          end
 
           # This second check is recorded for checking dependencies,
           # so executable are irrelevant at this point. If a piece

Library/Homebrew/exceptions.rb

@@ -115,14 +115,8 @@ end
 class IncompatibleCxxStdlibs < Homebrew::InstallationError
   def initialize(f, dep, wrong, right)
     super f, <<-EOS.undent
-    #{f} dependency #{dep} was built with the following
-    C++ standard library: #{wrong.type_string} (from #{wrong.compiler})
-
-    This is incompatible with the standard library being used
-    to build #{f}: #{right.type_string} (from #{right.compiler})
-
-    Please reinstall #{dep} using a compatible compiler.
-    hint: Check https://github.com/Homebrew/homebrew/wiki/C++-Standard-Libraries
+    #{f} dependency #{dep} was built with a different C++ standard
+    library (#{wrong.type_string} from #{wrong.compiler}). This could cause problems at runtime.
     EOS
   end
 end

Library/Homebrew/formula_installer.rb

@@ -123,7 +123,11 @@ class FormulaInstaller
 
         stdlibs = Keg.new(f.prefix).detect_cxx_stdlibs
         stdlib_in_use = CxxStdlib.new(stdlibs.first, MacOS.default_compiler)
+        begin
           stdlib_in_use.check_dependencies(f, f.recursive_dependencies)
+        rescue IncompatibleCxxStdlibs => e
+          opoo e.message
+        end
 
         stdlibs = Keg.new(f.prefix).detect_cxx_stdlibs :skip_executables => true
         tab = Tab.for_keg f.prefix