Needed for asprintf.

https://bugs.gentoo.org/870676
--- a/configure.in
+++ b/configure.in
@@ -8,6 +8,8 @@ AC_PROG_CC
 AC_PROG_RANLIB
 AC_PROG_INSTALL
 
+AC_USE_SYSTEM_EXTENSIONS
+
 dnl Checks for libraries.
 AC_CHECK_LIB(crypt, crypt)
 AC_CHECK_LIB(nsl, gethostbyname)