Can you be more specific about what you mean by "executable" and "library"?

In particular, please show the results of, where /path/to/python is replaced by the path to the python you have built or installed:

otool -L $(/path/to/python -c 'import _tkinter;print(_tkinter.__file__)')

Most likely the issue is that you need to use additional ./configure arguments rather than trying to modify CFLAGS et al:

  --with-tcltk-includes='-I...'
                          override search for Tcl and Tk include files
  --with-tcltk-libs='-L...'
                          override search for Tcl and Tk libs

or set up the path to pkg-config info for Homebrew's Tcl and Tk packages.  See the comments in setup.py for more info:

https://github.com/python/cpython/blob/main/setup.py#L1898