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>

meson.build

@@ -1797,8 +1797,7 @@ malloc = []
 if get_option('malloc') == 'system'
   has_malloc_trim = \
     get_option('malloc_trim').allowed() and \
-    cc.links('''#include <malloc.h>
-                int main(void) { malloc_trim(0); return 0; }''')
+    cc.has_function('malloc_trim', prefix: '#include <malloc.h>')
 else
   has_malloc_trim = false
   malloc = cc.find_library(get_option('malloc'), required: true)
@@ -1811,34 +1810,19 @@ if not has_malloc_trim and get_option('malloc_trim').enabled()
   endif
 endif
 
-# Check whether the glibc provides statx()
-
 gnu_source_prefix = '''
   #ifndef _GNU_SOURCE
   #define _GNU_SOURCE
   #endif
 '''
-statx_test = gnu_source_prefix + '''
-  #include <sys/stat.h>
-  int main(void) {
-    struct statx statxbuf;
-    statx(0, "", 0, STATX_BASIC_STATS, &statxbuf);
-    return 0;
-  }'''
 
-has_statx = cc.links(statx_test)
+# Check whether the glibc provides STATX_BASIC_STATS
 
-# Check whether statx() provides mount ID information
+has_statx = cc.has_header_symbol('sys/stat.h', 'STATX_BASIC_STATS', prefix: gnu_source_prefix)
 
-statx_mnt_id_test = gnu_source_prefix + '''
-  #include <sys/stat.h>
-  int main(void) {
-    struct statx statxbuf;
-    statx(0, "", 0, STATX_BASIC_STATS | STATX_MNT_ID, &statxbuf);
-    return statxbuf.stx_mnt_id;
-  }'''
+# Check whether statx() provides mount ID information
 
-has_statx_mnt_id = cc.links(statx_mnt_id_test)
+has_statx_mnt_id = cc.has_header_symbol('sys/stat.h', 'STATX_MNT_ID', prefix: gnu_source_prefix)
 
 have_vhost_user_blk_server = get_option('vhost_user_blk_server') \
   .require(targetos == 'linux',