XXX base gcc
Reordering needed to prefer long long because base-gcc doesn't recognize
  #pragma GCC diagnostic error "-Wincompatible-pointer-types"
and doesn't fail to compile the test case (gint64 should be long long).

Index: meson.build
--- meson.build.orig
+++ meson.build
@@ -1443,18 +1443,6 @@ endif
 # works.
 if long_long_size == long_size
   if cc.compiles('''#if defined(_AIX) && !defined(__GNUC__)
-                    #pragma options langlvl=stdc99
-                    #endif
-                    #pragma GCC diagnostic error "-Wincompatible-pointer-types"
-                    #include <stdint.h>
-                    #include <stdio.h>
-                    int main () {
-                      int64_t i1 = 1;
-                      long *i2 = &i1;
-                      return 1;
-                    }''', name : 'int64_t is long')
-    int64_t_typedef = 'long'
-  elif cc.compiles('''#if defined(_AIX) && !defined(__GNUC__)
                       #pragma options langlvl=stdc99
                       #endif
                       #pragma GCC diagnostic error "-Wincompatible-pointer-types"
@@ -1466,6 +1454,18 @@ if long_long_size == long_size
                         return 1;
                       }''', name : 'int64_t is long long')
     int64_t_typedef = 'long long'
+  elif cc.compiles('''#if defined(_AIX) && !defined(__GNUC__)
+                    #pragma options langlvl=stdc99
+                    #endif
+                    #pragma GCC diagnostic error "-Wincompatible-pointer-types"
+                    #include <stdint.h>
+                    #include <stdio.h>
+                    int main () {
+                      int64_t i1 = 1;
+                      long *i2 = &i1;
+                      return 1;
+                    }''', name : 'int64_t is long')
+    int64_t_typedef = 'long'
   endif
 endif