This is actually a bug in the Tcl configure script. Starting in Tcl 8.6.11, the lines:

    if test "$prefix/lib" != "$libdir"; then
        TCL_PACKAGE_PATH="${libdir} ${prefix}/lib"
    else
        TCL_PACKAGE_PATH="${prefix}/lib"
    fi

were changed to:

    if test "$prefix/lib" != "$libdir"; then
        TCL_PACKAGE_PATH="{${libdir}} {${prefix}/lib}"
    else
        TCL_PACKAGE_PATH="{${prefix}/lib}"
    fi

The extra braces propagate into the tclConfig.sh file, which then breaks the makefile.