tclDecls.h comes with a new redefinition which is in effect when compiling against Tcl stubs (-DUSE_TCL_STUBS) around line 3975:

#ifndef __cplusplus
#   undef Tcl_EventuallyFree
#   define Tcl_EventuallyFree \
           ((void (*)(void *,void *))(void *)(tclStubsPtr->tcl_EventuallyFree)) /* 132 */
#   undef Tcl_SetResult
#   define Tcl_SetResult \
           ((void (*)(Tcl_Interp *, char *, void *))(void *)(tclStubsPtr->tcl_SetResult)) /* 232 */
#endif

This should be disabled since Tcl_FreeProc does not match the declared prototype. I removed this setting and rebuild Tcl on Linux/Windows/macos without problems and I was able to compile extensions against the stub library.

I propose to remove this questionable redefinition to avoid compiler warnings about non-matching types for TCL_STATIC, TCL_DYNAMIC and friends.