while doing make install, ./tclsh gets invoked. however that assumes that the dynamic linker knows the location of libtcl8.6.so
if no previous installation of tcl is available on the host, and DESTDIR is used, this shared object cannot be found, the tclsh binary not started and subsequently the build breaks.
(the dynlinker searches in /lib or /usr/lib, but when DESTDIR is e.g. /opt/tcl, it won't find any shared objects there)
thus, LD_LIBRARY_PATH must be used when invoking the tclsh binary.
as a temporary workaround:
make install DESTDIR=xxx
makes the build succeed.