unicornscan: fixed compiling by passing -D_GNU_SOURCE in CFLAGS (see note)

Note:

Without passing -D_GNU_SOURCE the build fails with the error:
storage size of 'ccred' isn't known.

security/unicornscan/unicornscan.conf

@@ -2,7 +2,7 @@
 # This copyright note is auto-generated by ./scripts/Create-CopyPatch.
 #
 # Filename: package/.../unicornscan/unicornscan.conf
-# Copyright (C) 2008 The OpenSDE Project
+# Copyright (C) 2008 - 2011 The OpenSDE Project
 #
 # More information can be found in the files COPYING and README.
 #
@@ -13,3 +13,7 @@
 # --- SDE-COPYRIGHT-NOTE-END ---
 
 var_append makeinstopt ' ' "PREFIX=$root/$prefix"
+
+# without passing -D_GNU_SOURCE the build fails with the error:
+# storage size of 'ccred' isn't known
+var_append configprefix ' ' 'CFLAGS=-D_GNU_SOURCE'