[bug #9445] makefiles not safe for parallel build

[Nicola Pero]

Follow-up Comment #4, bug #9445 (project gnustep):

I have been thinking for a while about the new .NOTPARALLEL 
target in GNU make ... :-)

... maybe we could move all the file compilations into a separate
recursive make invocation, then let make go wild with 
parallelization on that sub-make invocation only. :-)

So, we keep all our "serial" features such as support for
before-all::, after-libgnustep-base-install:: targets, but
we parallelize the actual compilation.

The Instance invocation would do all the usual stuff, but
we'd replace the rule to build OBJ_FILES with a phony target
'compile-obj-files', which itself would fire up another
make instance.  That make instance would only execute the
rules to build OBJ_FILES and could be safely parallelized
(by omitting the .NOTPARALLEL: target).

Now this would allow parallel compilation (eg, if you
compile gnustep-base, which contains something like 100+
Objective-C files to compile, the compilation would happen
in parallel) which would presumably be faster on modern
machines; but it would add a make invocation per Instance, 
which would make things a little bit slower on older
machines.  I suppose it might be worth it ... if people
then actually use 'make -j4' to take advantage of it.

When I have a bit of time, I'll play with this.

Thanks

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