https://github.com/collectd/collectd/issues/1147

diff --git a/configure.ac b/configure.ac
index 6ee6e9a..a0e35b6 100644
--- a/configure.ac
+++ b/configure.ac
@@ -49,6 +49,7 @@ AC_PROG_LEX
 AC_PROG_YACC
 PKG_PROG_PKG_CONFIG
 
+AC_GNU_SOURCE
 AC_CHECK_PROG([have_protoc_c], [protoc-c], [yes], [no])
 AC_CHECK_HEADERS([google/protobuf-c/protobuf-c.h],
                  [have_protobuf_c_h="yes"],