The description about using -DPIKCHR_TCL in order to create a library for TCL could be accomanied by an entry in Makefile:

piktcl: pikchr.c  
        $(CC) -shared -o libpikchr.so $(CFLAGS) -DPIKCHR_TCL -I /usr/include/tcl8.6 pikchr.c -lm -L /usr/lib/x86_64-linux-gnu/ -ltcl8.6

This one works on Ubuntu 20.04

The created library can be loaded via the load command, but creating a pkgIndex.tcl file and using package require doesn't work.

The error is:

attempt to provide package pikchr 1.0 failed: no version of package pikchr provided  
while evaluating package require pikchr 1.0

I had created a pkgIndex.tcl file with this entry:

package ifneeded pikchr 1.0 [list load [file join $dir libpikchr.so]]

What has to be done to allow a package require. I cannot find any Tcl_PkgProvide call in the source code of pikchr.

I believe that the missing Tcl_PkgProvide (in the Pikchr_Init function) is the main issue here.

Can you check in a patch, Andreas?

I will try.