Tcl/tk not found when building Matplot 0.87.2 under Debian 3.10a for AMD64

cannot find tcl/tk headers. giving up.

    > /usr/lib/tcl8.4 and /usr/lib/tk8.4 both exist.

But to the tk *headers* exist? Perhaps you need to install the
tk8.4-dev and tcl8.4-dev packages?

peds-pc311:~/mpl> ls /usr/include/tcl8.4/
tclDecls.h tclPlatDecls.h tkDecls.h tkPlatDecls.h
tcl.h tcl-private tk.h tk-private

You also have the option of not building the tkagg backend, and using
gtk*, qtagg or wxagg.