Add cxxstdlib_check method to request changing C++ stdlib checking.

* In certain cases, a C++ software may result in linking to a different
  and incompatible C++ standard library than its dependencies and still
  works fine because it is by design. Examples include GCC, which will
  bootstrap itself and self-host after finish.

* Add a cxxstdlib_check method to formula to request changing the C++
  standard library checking. Currently using "cxxstdlib_check :skip"
  will let a formula skip such checking. This should only be used on
  rare occasions and be very careful.

Closes Homebrew/homebrew#23687.

Library/Homebrew/cxxstdlib.rb

@@ -33,14 +33,17 @@ class CxxStdlib
   end
 
   def check_dependencies(formula, deps)
-    deps.each do |dep|
-      # Software is unlikely to link against anything from its buildtime deps,
-      # so it doesn't matter at all if they link against different C++ stdlibs
-      next if dep.tags.include? :build
+    unless formula.cxxstdlib.include? :skip
+      deps.each do |dep|
+        # Software is unlikely to link against anything from its
+        # buildtime deps, so it doesn't matter at all if they link
+        # against different C++ stdlibs
+        next if dep.tags.include? :build
 
-      dep_stdlib = Tab.for_formula(dep.to_formula).cxxstdlib
-      if !compatible_with? dep_stdlib
-        raise IncompatibleCxxStdlibs.new(formula, dep, dep_stdlib, self)
+        dep_stdlib = Tab.for_formula(dep.to_formula).cxxstdlib
+        if !compatible_with? dep_stdlib
+          raise IncompatibleCxxStdlibs.new(formula, dep, dep_stdlib, self)
+        end
       end
     end
   end

Library/Homebrew/formula.rb

@@ -25,6 +25,10 @@ class Formula
 
   attr_accessor :local_bottle_path
 
+  # Flag for marking whether this formula needs C++ standard library
+  # compatibility check
+  attr_reader :cxxstdlib
+
   # Homebrew determines the name
   def initialize name='__UNKNOWN__', path=nil
     @name = name
@@ -52,6 +56,8 @@ class Formula
     @build = determine_build_options
 
     @pin = FormulaPin.new(self)
+
+    @cxxstdlib ||= Set.new
   end
 
   def set_spec(name)
@@ -643,6 +649,12 @@ class Formula
     end
   end
 
+  # Explicitly request changing C++ standard library compatibility check
+  # settings. Use with caution!
+  def cxxstdlib_check check_type
+    @cxxstdlib << check_type
+  end
+
   def self.method_added method
     case method
     when :brew