XXX push upstream
Needed for alloca(3).

Index: libgweather/meson.build
--- libgweather/meson.build.orig
+++ libgweather/meson.build
@@ -1,6 +1,6 @@
 add_project_arguments([
   '-DGWEATHER_COMPILATION',
-  '-D_XOPEN_SOURCE=700',
+  '-D_XOPEN_SOURCE_EXTENDED',
   '-D_DEFAULT_SOURCE',
 ], language: 'c')