Installing private header files to
/Library/Frameworks/Tcl.framework/Versions/8.6/PrivateHeaders/
ERROR: version conflict for package "Tcl": have 8.5.9, need 8.6
If running this script from 'make html', set the NATIVE_TCLSH environment
variable to point to an installed tclsh8.6 (or the equivalent tclsh86.exe
on Windows).
make: *** [html-tcl] Error 1

sudo make install NATIVE_TCLSH=/usr/local/bin/tclsh8.6