configure: Move preadv check to meson.build

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

configure

@@ -3531,19 +3531,6 @@ if compile_prog "" "" ; then
   iovec=yes
 fi
 
-##########################################
-# preadv probe
-cat > $TMPC <<EOF
-#include <sys/types.h>
-#include <sys/uio.h>
-#include <unistd.h>
-int main(void) { return preadv(0, 0, 0, 0); }
-EOF
-preadv=no
-if compile_prog "" "" ; then
-  preadv=yes
-fi
-
 ##########################################
 # fdt probe
 
@@ -5748,9 +5735,6 @@ fi
 if test "$iovec" = "yes" ; then
   echo "CONFIG_IOVEC=y" >> $config_host_mak
 fi
-if test "$preadv" = "yes" ; then
-  echo "CONFIG_PREADV=y" >> $config_host_mak
-fi
 if test "$membarrier" = "yes" ; then
   echo "CONFIG_MEMBARRIER=y" >> $config_host_mak
 fi