lilypond: add option to build docs

Closes Homebrew/homebrew#14946.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/lilypond.rb

@@ -26,6 +26,8 @@ class Lilypond < Formula
 
   env :userpaths
 
+  option 'with-doc', "Build documentation in addition to binaries (may require several hours)."
+
   depends_on TexInstalled.new
   depends_on 'pkg-config' => :build
   depends_on 'gettext'
@@ -37,6 +39,15 @@ class Lilypond < Formula
   depends_on 'texinfo'
   depends_on :x11
 
+  # Assert documentation dependencies if requested.
+  if build.include? 'with-doc'
+    depends_on 'netpbm'
+    depends_on 'imagemagick'
+    depends_on 'docbook'
+    depends_on 'dblatex' => :python
+    depends_on 'texi2html'
+  end
+
   skip_clean :all
 
   fails_with :clang do
@@ -52,6 +63,12 @@ class Lilypond < Formula
     # Separate steps to ensure that lilypond's custom fonts are created.
     system 'make all'
     system "make install"
+
+    # Build documentation if requested.
+    if build.include? 'with-doc'
+      system "make doc"
+      system "make install-doc"
+    end
   end
 
   def test