@User - just an update, using -std=c90 does eliminate the C99 references in the errors, but I still get the errors marked [-Werror,-Wimplicit-function-declaration]. Looking at the clang documentation at llvm.org, there isn't even a reference to -Werror, much less how to disable it. Apparently this is their strong-arm way of discouraging the practice... I'm using the Clang12 bundled in Xcode. I tried using the blunt instrument of "make -i". It compiled object files until it ran into this problem: --- linking libtcltk.o --- making Tcl shared libraries make[2]: * No rule to make target `../bplane/libbplane.o', needed by `tclmagic.dylib'. Stop. bash-3.2$