meson: Check header usability

meson: Check header usability

This fixes cross-building in some scenarios.

Specifically, when building for armv7l on x86_64, has_header()
will see the x86_64 version of the linux/kmv.h header and
consider it to be usable. Later, when an attempt is made to
actually include it, the compiler will quickly realize that
things can't quite work.

The reason why we haven't hit this in our CI is that we only ever
install the foreign version of header files. When building the
Debian package, however, some of the Debian-specific tooling will
bring in the native version of the Linux headers in addition to
the foreign one, causing meson to misreport the header's
availability status.

Checking for actual usability, as opposed to mere presence, of
headers is enough to make things work correctly in all cases.

The meson documentation recommends using has_header() instead of
check_header() whenever possible for performance reasons, but
while testing this change on fairly old and underpowered hardware
I haven't been able to measure any meaningful slowdown.

https://bugs.debian.org/1024504

Suggested-by: Helmut Grohne <helmut@subdivi.de>
Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>