Fix build on macOS High Sierra and later
https://github.com/FreeRDP/FreeRDP/issues/4160
https://github.com/FreeRDP/FreeRDP/commit/6309459839e49129487a38d7c6aa16c2991f57ea
--- config.h.in.orig	2017-07-27 07:04:21.000000000 -0500
+++ config.h.in	2017-10-02 18:50:40.000000000 -0500
@@ -13,7 +13,11 @@
 
 /* Include files */
 #cmakedefine HAVE_FCNTL_H
+#if defined(__APPLE__) && !defined(__IOS__)
+#cmakedefine01 HAVE_UNISTD_H
+#else
 #cmakedefine HAVE_UNISTD_H
+#endif
 #cmakedefine HAVE_STDINT_H
 #cmakedefine HAVE_INTTYPES_H
 #cmakedefine HAVE_SYS_MODEM_H