Create an initial compatibility shim for unistd.h.

src/include/compat/unistd.h

@@ -0,0 +1,27 @@
+/* Compatibility shim for <unistd.h>
+ *
+ * Copyright (C) 2016 Reece H. Dunn
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see: <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef UNISTD_H_COMPAT_SHIM
+#define UNISTD_H_COMPAT_SHIM
+
+#if defined(HAVE_UNISTD_H)
+#pragma GCC system_header // Silence "warning: #include_next is a GCC extension"
+#include_next <unistd.h>
+#endif
+
+#endif