In commit ac988cf we made sure to undefine the CONST/VOID macros left
defined by `tcl.h`, but this in turn makes it an issue to include
additional Tcl headers later on (see issue #4808).
One way out is to avoid a global `tcl.h` include. In the process we drop
support for Tcl-enabled MXE builds, which were likely broken anyway due
to the additional Tcl APIs used from `tclapi.cc`.