Builds is not entirely successful if there were warnings

Print a last line with a yellow warning if there was a STDERR in
at least one executed command.

For example:

    $ buildsome
    Parsed makefile: "Buildsome.mk" (took 0.227192ssec)
    T000 Building: "default"
    T010 REPLAY for target ["out/main.o"] (originally) took 0.037523s seconds, STDERR follows
    main.c:89:6: warning: function declaration isn't a prototype [-Wstrict-prototypes]
     void warning_generator() {};
      ^
    T000 Build Successful (but with STDERR): 0.271871s seconds total.