+# If autoconf reports a warning
+# warning: AC_COMPILE_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS
+# or warning: AC_RUN_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS
+# the fix is
+# 1) to ensure that AC_USE_SYSTEM_EXTENSIONS is never directly invoked
+# but always AC_REQUIREd,
+# 2) to ensure that for each occurrence of
+# AC_REQUIRE([AC_USE_SYSTEM_EXTENSIONS])
+# or
+# AC_REQUIRE([gl_USE_SYSTEM_EXTENSIONS])
+# the corresponding gnulib module description has 'extensions' among
+# its dependencies. This will ensure that the gl_USE_SYSTEM_EXTENSIONS
+# invocation occurs in gl_EARLY, not in gl_INIT.
+