Add url_for_version and depends_on('libiconv') (#3555)

* Add url_for_version and depends_on('libiconv')

Fetching is broken.  See https://github.com/LLNL/spack/issues/3554.

libgd, when used in gnuplot at least, needs a dependency
on libiconv.

* Undo change from debugging pokery

var/spack/repos/builtin/packages/libgd/package.py

@@ -52,6 +52,11 @@ class Libgd(AutotoolsPackage):
     depends_on('gettext', type='build')
     depends_on('pkg-config', type='build')
 
+    depends_on('libiconv')
     depends_on('libpng')
     depends_on('libtiff')
     depends_on('fontconfig')
+
+    def url_for_version(self, version):
+        url = "https://github.com/libgd/libgd/releases/download/gd-{0}/libgd-{0}.tar.gz"
+        return url.format(version)