Re: adding specific C flags for a SINGLE source file

[Sander Niemeijer]

> > user_CFLAGS=$CFLAGS
> > AC_PROG_CC
> > if test "x$user_CFLAGS" = x; then
> >   # If the user didn't specify CFLAGS, then CFLAGS contains
> >   # a subset of -g -O2 selected by AC_PROG_CC.  This is not
> >   # a user setting, and we want to be able to override this
> >   # locally in our rules, so put these flags in a separate
> >   # variable and empty CFLAGS.
> >   AC_SUBST([DEFAULTFLAGS], [$CFLAGS])
> >   CFLAGS=
> > fi
> >
> > and in Makefile.am use
> >   foo_CFLAGS = $(DEFAULTFLAGS)
> > and
> >   libfoo_a_CFLAGS = $(DEFAULTFLAGS) $(O3)
> > as appropriate.
> >
> > ($(O3) being the Makefile variable that contains -O3 if the compiler
> > support it).

I just noticed something else I don't like about this approach. If you 
do it this way a user will never be able to change CFLAGS and still be 
able to only provide specific optimization flags for libfoo. For 
example, it won't be possible to have CFLAGS='-g -O0' but still have 
libfoo compiled with e.g. '-O2'.

Although I agree that users should always be able to have the last say, 
I do not think that a user provided CFLAGS setting should override 
_all_. If it were possible to have '$(CFLAGS) $(O3)' for libfoo (which 
is not the case in the code example above), then a user would still be 
able to have the last say if the O3 variable was an AC_ARG_VAR. He 
would be able to change the globally default flags by providing his own 
CFLAGS, but in addition would also still be able to tune the 
optimization of _just_ libfoo by supplying his own values for O3.

With the example above, a user can only tune both global CFLAGS _and_ 
libfoo optimization flags if he used a 'make DEFAULTFLAGS=... O3=...', 
which is in my opinion not an ideal approach (it would defeat the 
purpose of having a CFLAGS variable).

Best regards,
Sander