uapi libc compat: add fallback for unsupported libcs

uapi libc compat: add fallback for unsupported libcs

libc-compat.h aims to prevent symbol collisions between uapi and libc
headers for each supported libc. This requires continuous coordination
between them.

The goal of this commit is to improve the situation for libcs (such as
musl) which are not yet supported and/or do not wish to be explicitly
supported, while not affecting supported libcs. More precisely, with
this commit, unsupported libcs can request the suppression of any
specific uapi definition by defining the correspondings _UAPI_DEF_*
macro as 0. This can fix symbol collisions for them, as long as the
libc headers are included before the uapi headers. Inclusion in the
other order is outside the scope of this commit.

All infrastructure in order to enable this fallback for unsupported
libcs is already in place, except that libc-compat.h unconditionally
defines all _UAPI_DEF_* macros to 1 for all unsupported libcs so that
any previous definitions are ignored. In order to fix this, this commit
merely makes these definitions conditional.

This commit together with the musl libc commit

http://git.musl-libc.org/cgit/musl/commit/?id=04983f2272382af92eb8f8838964ff944fbb8258

fixes for example the following compiler errors when <linux/in6.h> is
included after musl's <netinet/in.h>:

./linux/in6.h:32:8: error: redefinition of 'struct in6_addr'
./linux/in6.h:49:8: error: redefinition of 'struct sockaddr_in6'
./linux/in6.h:59:8: error: redefinition of 'struct ipv6_mreq'

The comments referencing glibc are still correct, but this file is not
only used for glibc any more.

Signed-off-by: Felix Janda <felix.janda@posteo.de>
Reviewed-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>