Add formula for pdflib-lite.

pdflib-lite is required to enable Gnuplot's PDF terminal.

While not free (as in speach) software, pdflib-lite is available for
non-commercial (i.e., academic and personal) use free of charge. A license
reminder is printed at the end of the build.

pdflib is also provided by MacPorts; a workaround from their portfile was
adapted to this formula to get the build to go through.

Formula/pdflib-lite.rb

@@ -0,0 +1,31 @@
+require 'formula'
+
+class PdflibLite <Formula
+  url 'http://www.pdflib.com/binaries/PDFlib/704/PDFlib-Lite-7.0.4p4.tar.gz'
+  homepage 'http://www.pdflib.com/download/free-software/pdflib-lite/'
+  md5 '1765de9c5f2b6c5769a2160eaa911e9a'
+  version "7.0.4p4"
+
+  def install
+    # SL 10.6.1 LLVM crashes with an internal compiler error on this version.
+    ENV.gcc_4_2
+
+    # Without the following substituion, pdflib-lite runs into weird
+    # build errors due to bad interactions with the TIFF headers.
+    # This workaround comes from the MacPorts.org portfile for pdflib.
+    ENV['CPPFLAGS'] = "-isystem#{prefix}"
+
+    system "./configure", "--prefix=#{prefix}", "--disable-debug", "--disable-dependency-tracking",
+           "--without-java", "--without-perl", "--without-py", "--without-tcl",
+           "--without-ruby"
+    system "make"
+    system "make install"
+    
+    def caveats; <<-EOM
+pdflib-lite is not open source software; usage restrictions apply! Be sure to
+understand and obey the license terms, which can be found at:
+http://www.pdflib.com/products/pdflib-family/pdflib-lite/pdflib-lite-licensing/
+      EOM
+    end
+  end
+end