I am not sure if this is appropriate to be put on this. Currently my system has installed tcl/tk 8.6 (Which might be another bug, since I have disabled ~amd64 on these two packages, in theory the stable(8.5) version should be selected, but they are emerged anyway), and emerging brlcad produces this error:
/usr/include/tk.h:21:3: error: #error Tk 8.6 must be compiled with tcl.h from Tcl 8.6 or better
Complete log in attachment. Note that the log file is a slightly modified ebuild to build brlcad-7.24.0, but the portage version produces the same error.

It is very possible that this is an upstream bug, but there might be a workaround on this.