Simplify macro handling in tclOO*Decls.h, just as already done in "novem" [0c37ab8944], itcl*Decls.h and tdbc*Decls.h. 
This doesn't change the way symbols are exported. This simplifications were already present in the Tcl 8.6.2 headers, but those were buggy when tclOO was linked in statically without using stubs.