ploticus: add prefabs to installation

The existing formula for Ploticus 2.42 omits the installation of
the "prefabs" directory, which most users will need.

This patch ensures that the prefabs are installed into pkgshare,
and that ploticus knows where to find them without having to set
the PREFABS_DIR environment variable.

Closes Homebrew/homebrew#48296.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>

Formula/ploticus.rb

@@ -4,6 +4,7 @@ class Ploticus < Formula
   url "https://downloads.sourceforge.net/project/ploticus/ploticus/2.42/ploticus242_src.tar.gz"
   version "2.42"
   sha256 "3f29e4b9f405203a93efec900e5816d9e1b4381821881e241c08cab7dd66e0b0"
+  revision 1
 
   bottle do
     cellar :any
@@ -18,10 +19,17 @@ class Ploticus < Formula
     # Use alternate name because "pl" conflicts with OS X "pl" utility
     args=["INSTALLBIN=#{bin}",
           "EXE=ploticus"]
+    inreplace "src/pl.h", /#define\s+PREFABS_DIR\s+""/, "#define PREFABS_DIR \"#{pkgshare}\""
     system "make", "-C", "src", *args
     # Required because the Makefile assumes INSTALLBIN dir exists
     bin.mkdir
     system "make", "-C", "src", "install", *args
+    pkgshare.install Dir["prefabs/*"]
+  end
+
+  def caveats; <<-EOS.undent
+    Ploticus prefabs have been installed to #{opt_pkgshare}
+  EOS
   end
 
   test do