Run command passed to if_not_there directly from Makefile.direct

Run command passed to if_not_there directly from Makefile.direct

Issue #199 (bdwgc).

if_not_there tool is modified to accept a single argument (file name).

* Makefile.direct (base_lib, cords, test_cpp, c++, mach_dep.o, mark_rts.o,
cord/cordtest, cord/de, gctest): Replace "if_not_there <file> <command>"
with "if_not_there <file> || <command>".
* Makefile.direct (c++): Add dependency on $(UTILS).
* Makefile.direct (gctest): Remove double spaces.
* tools/if_not_there.c: Update header comment.
* tools/if_not_there.c (main): Allow both 2 and 3 command-line arguments;
return 2 if the file does not exist and argc is 2; update the printed
usage message.