The code already checked for the WITH_AIO macro, and almost all
configurable options in libiio and IIOD use the "WITH" prefix.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>

Old OSes (CentOS 7, Ubuntu 16.04) require s6_addr32 to be defined for
the IN6_IS_ADDR_LINKLOCAL() macro to work.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>

This function is not used in libiio, but is used by IIOD.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>

We do not want to compile with GNU-specific extensions, as we want the
code to be as portable as possible.

Thanksfully we don't actually need _GNU_SOURCE to be defined, removing
it only triggers an error in network-unix.c about an undefined
structure, which we can fix by including the right header.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>

Instead of making build options available according to whether or not a
library is found, only search for the library if the option has been
enabled.

This change may temporarily break the auto-builders (which will need to
be updated), but it makes the build *reproductible*, and that's very
important.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>

Disable serial backend, enable ZSTD support.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>

Disable serial and USB backends.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>

Enable ZSTD support, except on Ubuntu 16.04 where it doesn't build, and
explicitely set the build options as it is required now.

Note that the serial backend is disabled on Ubuntu 16.04 as its version
of libserialport is too old, and USB support in IIOD is disabled in
CentOS 7 as its kernel headers are too old.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>