meson: unify linux/stat.h check with other checks and use _GNU_SOURCE

Using _GNU_SOURCE is better because that's how we include the headers in the
actual build, and some headers define different stuff when it is defined.
sys/stat.h for example defines 'struct statx' conditionally.

meson.build

@@ -421,11 +421,9 @@ decl_headers = '''
 #include <uchar.h>
 #include <linux/ethtool.h>
 #include <linux/fib_rules.h>
-#include <linux/stat.h>
 #include <sys/stat.h>
 '''
 # FIXME: key_serial_t is only defined in keyutils.h, this is bound to fail
-# FIXME: these should use -D_GNU_SOURCE, since that is defined at build time
 
 foreach decl : ['char16_t',
                 'char32_t',
@@ -436,13 +434,23 @@ foreach decl : ['char16_t',
                ]
 
         # We get -1 if the size cannot be determined
-        have = cc.sizeof(decl, prefix : decl_headers) > 0
+        have = cc.sizeof(decl, prefix : decl_headers, args : '-D_GNU_SOURCE') > 0
+
+        if decl == 'struct statx'
+                if have
+                        want_linux_stat_h = false
+                else
+                        have = cc.sizeof(decl,
+                                         prefix : decl_headers + '#include <linux/stat.h>',
+                                         args : '-D_GNU_SOURCE') > 0
+                        want_linux_stat_h = have
+                endif
+        endif
+
         conf.set10('HAVE_' + decl.underscorify().to_upper(), have)
 endforeach
 
-conf.set10('HAVE_STRUCT_STATX_IN_SYS_STAT_H', cc.sizeof('struct statx', prefix : '''
-#include <sys/stat.h>
-''', args : '-D_GNU_SOURCE') > 0)
+conf.set10('WANT_LINUX_STAT_H', want_linux_stat_h)
 
 foreach decl : [['IFLA_INET6_ADDR_GEN_MODE',         'linux/if_link.h'],
                 ['IN6_ADDR_GEN_MODE_STABLE_PRIVACY', 'linux/if_link.h'],

src/basic/missing.h

@@ -24,7 +24,7 @@
 #include <uchar.h>
 #include <unistd.h>
 
-#if !HAVE_STRUCT_STATX_IN_SYS_STAT_H
+#if WANT_LINUX_STAT_H
 #include <linux/stat.h>
 #endif