I've found a flaw in the build of 8.6 tclkit using Pat Thoyt kitgen from
github. The target build/tcl doesn't install standard headers because of
a change in the tcl makefiles. The target install-libraries in the 8.5
makefile installs the standard headers, while the 8.6 makefile doesn't.
There's a new target, install-headers, instead.

My build was failing because kitInit.c was including an 8.5 tcl.h from
/usr/local/include instead of the correct one from build/include (which
wasn't there).

I haven't managed to find the perfect fix. I just added the target
install-headers to Makefile like this -

PRIV       = install-headers install-private-headers