C++ stdlib check: don't check executables for deps

This avoids some possible false positives, as happens with, e.g., qt4.

Library/Homebrew/build.rb

@@ -164,6 +164,9 @@ class Build
         begin
           f.install
 
+          # This first test includes executables because we still
+          # 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
@@ -173,6 +176,13 @@ class Build
           # incompatibility.
           stdlib_in_use.check_dependencies(f, deps)
 
+          # This second check is recorded for checking dependencies,
+          # so executable are irrelevant at this point. If a piece
+          # of software installs an executable that links against libstdc++
+          # and dylibs against libc++, libc++-only dependencies can safely
+          # link against it.
+          stdlibs = Keg.new(f.prefix).detect_cxx_stdlibs :skip_executables => true
+
           Tab.create(f, ENV.compiler, stdlibs.first,
             Options.coerce(ARGV.options_only)).write
         rescue Exception => e

Library/Homebrew/formula_installer.rb

@@ -125,6 +125,7 @@ class FormulaInstaller
         stdlib_in_use = CxxStdlib.new(stdlibs.first, MacOS.default_compiler)
         stdlib_in_use.check_dependencies(f, f.recursive_dependencies)
 
+        stdlibs = Keg.new(f.prefix).detect_cxx_stdlibs :skip_executables => true
         tab = Tab.for_keg f.prefix
         tab.poured_from_bottle = true
         tab.tabfile.delete if tab.tabfile

Library/Homebrew/keg_fix_install_names.rb

@@ -66,10 +66,11 @@ class Keg
   # lib/, and ignores binaries and other mach-o objects
   # Note that this doesn't attempt to distinguish between libstdc++ versions,
   # for instance between Apple libstdc++ and GNU libstdc++
-  def detect_cxx_stdlibs
+  def detect_cxx_stdlibs(opts={:skip_executables => false})
     results = Set.new
 
     mach_o_files.each do |file|
+      next if file.mach_o_executable? && opts[:skip_executables]
       dylibs = file.dynamically_linked_libraries
       results << :libcxx unless dylibs.grep(/libc\+\+.+\.dylib/).empty?
       results << :libstdcxx unless dylibs.grep(/libstdc\+\+.+\.dylib/).empty?