Move the preadv availability check to meson.build.  This is what we
want to be doing for host-OS-feature-checks anyway, but it also fixes
a problem with building for macOS with the most recent XCode SDK on a
Catalina host.

On that configuration, 'preadv()' is provided as a weak symbol, so
that programs can be built with optional support for it and make a
runtime availability check to see whether the preadv() they have is a
working one or one which they must not call because it will
runtime-assert.  QEMU's configure test passes (unless you're building
with --enable-werror) because the test program using preadv()
compiles, but then QEMU crashes at runtime when preadv() is called,
with errors like:

  dyld: lazy symbol binding failed: Symbol not found: _preadv
    Referenced from: /Users/pm215/src/qemu/./build/x86/tests/test-replication
    Expected in: /usr/lib/libSystem.B.dylib

  dyld: Symbol not found: _preadv
    Referenced from: /Users/pm215/src/qemu/./build/x86/tests/test-replication
    Expected in: /usr/lib/libSystem.B.dylib

Meson's own function availability check has a special case for macOS
which adds '-Wl,-no_weak_imports' to the compiler flags, which forces
the test to require the real function, not the macOS-version-too-old
stub.

So this commit fixes the bug where macOS builds on Catalina currently
require --disable-werror.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20210126155846.17109-1-peter.maydell@linaro.org