Allow missing libraries

Library/Homebrew/formula.rb

@@ -1131,6 +1131,13 @@ class Formula
     end
   end
 
+  # Whether this {Formula} is allowed to have broken linkage.
+  # Defaults to false.
+  # @return [Boolean]
+  def allow_missing_libs?
+    false
+  end
+
   # Whether this {Formula} is deprecated (i.e. warns on installation).
   # Defaults to false.
   # @method deprecated?
@@ -2596,6 +2603,10 @@ class Formula
       @skip_clean_paths ||= Set.new
     end
 
+    def allow_missing_libs
+      define_method(:allow_missing_libs?) { true }
+    end
+
     # Software that will not be symlinked into the `brew --prefix` will only
     # live in its Cellar. Other formulae can depend on it and then brew will
     # add the necessary includes and libs (etc.) during the brewing of that

Library/Homebrew/linkage_checker.rb

@@ -59,14 +59,20 @@ class LinkageChecker
     display_items "Broken dependencies", @broken_deps, puts_output: puts_output
     display_items "Unwanted system libraries", @unwanted_system_dylibs, puts_output: puts_output
     display_items "Conflicting libraries", @version_conflict_deps, puts_output: puts_output
-    puts "No broken library linkage" unless broken_library_linkage?
+    if got_library_issues?
+      puts "Broken library linkage ignored" if @formula.allow_missing_libs?
+    else
+      puts "No broken library linkage"
+    end
   end
 
   def broken_library_linkage?
-    !@broken_dylibs.empty? ||
-      !@broken_deps.empty? ||
-      !@unwanted_system_dylibs.empty? ||
-      !@version_conflict_deps.empty?
+    !@formula.allow_missing_libs? && got_library_issues?
+  end
+
+  def got_library_issues?
+    issues = [@broken_dylibs, @broken_deps, @unwanted_system_dylibs, @version_conflict_deps]
+    issues.any?(&:present?)
   end
 
   private