[siddhesh/is_in_module] Fix SHLIB_COMPAT -Wundef warning

[Siddhesh Poyarekar <siddhesh at redhat dot com>]

Hi,

I've pushed the bits I've been working on for the SHLIB_COMPAT Wundef
warning to a private branch siddhesh/is_in_module.  I need to do some
more verification (and write ChangeLogs!) before I post the changes as
a patchset.  All patches barring one result in no changes[1] in
generated code, so I reckon they ought to be safe for inclusion if the
general idea is acceptable.

My approach with this patchset has been to replace the IS_IN_*,
NOT_IN_libc and IN_LIB macros with a single macro function
IS_IN_MODULE which is an accessor for the IN_MODULE macro, which is
always defined as MODULE_libc (actually MODULE_libc + MODULE_libs;
MODULE_libs is an indicator that the module is a core library,
i.e. libc, libm, librt, etc.) by default and explicitly overridden on
the commandline (using -UIN_MODULE and -DIN_MODULE=...).  It also
seemed to also improve readability of the macros to me.

I'd love to get feedback from arch maintainers on this branch since I
have only verified x86_64 builds and nothing else.

Thanks,
Siddhesh

[1] I learnt that nscd_stat burns a timestamp into the code, resulting
    in a change in code with every build.  That change crops up in
    every build.

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