some build.rs deduppage and improvements

* handles some a case with lib tinfo building like when setting
TINFOW_NO_PKG_CONFIG=1 env. var
* dedup some common code so changes won't have to be made in two/more places
* added some todos/fixmes
* made source for chtype_size.c not need escaping inside build.rs via br#"..."#
* unified cargo:rerun-if-env-changed={} under one function: watch_env_var()
* ...

* FIXME: this is currently now broken: cargo build --features=test_build_rs_of_ncurses_rs
  I'll probably force push a fix and remove this message soon.