A couple of minor build system fixes

meson.build

@@ -14,10 +14,7 @@ thread_dep = dependency('threads', required: false)
 
 conf_data = configuration_data()
 
-cc = meson.get_compiler('c')
-
-has_reallocarray = cc.has_function('reallocarray')
-if cc.has_function('reallocarray')
+if cc.has_function('reallocarray', prefix: '#define _GNU_SOURCE\n#include <stdlib.h>')
   conf_data.set('HAVE_REALLOCARRAY', 1)
 endif