Omit post-install warning unless formula defines it

Closes Homebrew/homebrew#34744.

Library/Homebrew/formula.rb

@@ -333,6 +333,10 @@ class Formula
   # Can be overridden to run commands on both source and bottle installation.
   def post_install; end
 
+  def post_install_defined?
+    method(:post_install).owner == self.class
+  end
+
   # tell the user about any caveats regarding this package, return a string
   def caveats; nil end
 

Library/Homebrew/formula_installer.rb

@@ -383,7 +383,7 @@ class FormulaInstaller
     link(keg)
     fix_install_names(keg) if OS.mac?
 
-    if build_bottle?
+    if build_bottle? && formula.post_install_defined?
       ohai "Not running post_install as we're building a bottle"
       puts "You can run it manually using `brew postinstall #{formula.name}`"
     else