[tlbuild] interact.cc and getline insanity

[Karl Berry]

FWIW, you can use 
  gcc -E -dM - </dev/null
to get a list of all predefined symbols in GCC.

Of course that doesn't help for symbols defined in #include files.  For
that, I use
  gcc -E -dD foo.c -o hello.cpp
which outputs all the #define's in sequence with the output from cpp,
so you can see exactly what is #defined where.

karl