Fix semantics of empty string in mkfiles.pl "!cflags".

Fix semantics of empty string in mkfiles.pl "!cflags".

Previously, if you tried to set the special cflags for an object file
to the empty string, mkfiles.pl would normalise that to the string
"1". I'm not entirely sure why - that line of code was added without
explanation in commit 64150a5ef which brought in that directive in the
first place - but I have to guess that it was left over from some
earlier design iteration in which I hadn't quite decided whether I was
going to need a string or a boolean to separate version.o from other
objects.

Of course, setting an object's cflags to "" is a bit of a weird thing
to want to do anyway - why not just leave them unset? But in fact I've
now thought of something useful for it to do: this commit arranges
that setting cflags="" has the effect (in the 'am' makefile type) of
separating the object out into its own little automake library but not
actually giving that library any separate cflags. And the point of
_that_, in turn, will be that then you can add cflags to it
_conditionally_ in a "!begin am" snippet, e.g. conditionalised on
something in configure.