meson: Avoid implicit declaration of absent functions

While detecting a presence of a function via 'cc.links()'
gives desired result (i.e. detects whether function is present),
it also produces a warning on systems where the function is not
present (into meson-log.txt), e.g.:

  qemu.git/build/meson-private/tmph74x3p38/testfile.c:2:34: \
  warning: implicit declaration of function 'malloc_trim' [-Wimplicit-function-declaration]

And some distributions (e.g. Gentoo) parse the meson log and
consider these erroneous because it can lead to feature
misdetection (see [1]).

We can check whether given function exists via
'cc.has_function()' or whether STATX_* macros exist via
'cc.has_header_symbol()'.

1: https://wiki.gentoo.org/wiki/Modern_C_porting
Resolves: https://bugs.gentoo.org/898810


Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Message-Id: <8e02776d18595a1c575c90a189ff65f1785f76ca.1685442612.git.mprivozn@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>