Error: tcl.h not found (no such file or directory)

Once you have a Tcl dev package installed, you'll then possibly come across the code expecting tcl.h to be in /usr/include/, but in order to facilitate multiple versions of Tcl being installed, Ubuntu places tcl.h in /usr/include/tcl/ - note the extra directory level.

Replacing: #include <tcl.h> with #include <tcl/tcl.h> in the source code you're trying to build should get around this.

This means you're missing the tcl development files. The easiest way to check is to go to: http://packages.ubuntu.com/ and put "tcl.h" in the "search contents of packages" field. If you do this you'll find that tcl.h can be found in a number of packages you'll want to install one of those:

sudo apt-get install tcl8.6-dev