CxxStdlib: don't check buildtime deps

Otherwise it's possible that certain buildtime dependencies, that won't
be linked against, will block a build.

See Homebrew/homebrew#23115.

Library/Homebrew/cxxstdlib.rb

@@ -34,6 +34,10 @@ class CxxStdlib
 
   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
+
       dep_stdlib = Tab.for_formula(dep.to_formula).cxxstdlib
       if !compatible_with? dep_stdlib
         raise IncompatibleCxxStdlibs.new(formula, dep, dep_stdlib, self)