macOS: add a workaround to build with newer SDKs
SDK 10.13 (and possibly olders as well) need HAVE_UNISTD_H to be set to
a value if available otherwise the following error is thrown:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk/usr/include/dispatch/dispatch.h:38:45:
error: expected value in expression #if !defined(HAVE_UNISTD_H) || HAVE_UNISTD_H
/* Include files */
#cmakedefine HAVE_FCNTL_H
+#if defined(__APPLE__) && !defined(__IOS__)
+#cmakedefine01 HAVE_UNISTD_H
+#else
#cmakedefine HAVE_UNISTD_H
+#endif
#cmakedefine HAVE_INTTYPES_H
#cmakedefine HAVE_SYS_MODEM_H
#cmakedefine HAVE_SYS_FILIO_H
