gnuplot: fixed broken package and add variants (#3185)

* gnuplot: fix conflict in header via a simple patch; add variants

* gtkplus: fix missing dependency

* wx: fix build on macOS; switch to AutotoolsPackage

* gnuplot: add missing dependencies

* wx: put back parallel build

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

@@ -36,6 +36,8 @@ class Gtkplus(AutotoolsPackage):
 
     variant('X', default=False, description="Enable an X toolkit")
 
+    depends_on('pkg-config', type='build')
+
     depends_on("atk")
     depends_on("gdk-pixbuf")
     depends_on("glib")