Success: math/gnuplot v4.6.1
bsd.wx.mk: Add wxX-common when wxX is requested

These are the headers that go along with the library.
They are already on a dirty environment because they are needed to build
wxX, but they weren't there when building from packages.  Update the
bsd.wx.mk to automatically include headers when library is requested.

(Fixes math/gnuplot)
math/gnuplot: fix (and add readline functionality)