Create an initial compatibility shim for unistd.h.

src/include/compat/unistd.h

+/* 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

src/libespeak-ng/compiledata.c

 #include "spect.h"
 
 #include <sys/stat.h>
-#ifdef PLATFORM_POSIX
 #include <unistd.h>
-#endif
 
 typedef struct {
 	unsigned int value;

src/libespeak-ng/espeak_command.h

 #ifndef ESPEAK_COMMAND_H
 #define ESPEAK_COMMAND_H
 
-#ifndef PLATFORM_WINDOWS
 #include <unistd.h>
-#endif
 
 #ifdef __cplusplus
 extern "C"

src/libespeak-ng/event.c

 
 #include "config.h"
 
-#ifndef PLATFORM_WINDOWS
 #include <unistd.h>
-#endif
 #include <assert.h>
 #include <string.h>
 #include <stdlib.h>

src/libespeak-ng/fifo.c

 
 #include "config.h"
 
-#ifndef PLATFORM_WINDOWS
 #include <unistd.h>
-#endif
 #include <assert.h>
 #include <string.h>
 #include <stdlib.h>

src/libespeak-ng/readclause.c

 #include "voice.h"
 #include "translate.h"
 
-#ifdef PLATFORM_POSIX
 #include <unistd.h>
-#endif
 
 #include <locale.h>
 #define N_XML_BUF   500

src/libespeak-ng/speech.c

 #include "speech.h"
 
 #include <sys/stat.h>
+#include <unistd.h>
 #ifdef PLATFORM_WINDOWS
 #include <fcntl.h>
 #include <io.h>
 #include <windows.h>
 #include <winreg.h>
-#else  /* PLATFORM_POSIX */
-#include <unistd.h>
 #endif
 
 #include "phoneme.h"

src/libespeak-ng/wave.c

 
 #ifdef PLATFORM_WINDOWS
 #include <windows.h>
-#else
-#include <unistd.h>
 #endif
+#include <unistd.h>
 #include "wave.h"
 
 #ifdef NEED_STRUCT_TIMESPEC

src/libespeak-ng/wave_pulse.c

 #include <time.h>
 #include <pulse/pulseaudio.h>
 #include <pthread.h>
-
-#ifndef PLATFORM_WINDOWS
 #include <unistd.h>
-#endif
+
 #include "wave.h"
 
 enum {

src/speak-ng.c

 #include <io.h>
 #include <windows.h>
 #include <winreg.h>
-#else
-#include <unistd.h>
 #endif
 #endif
 
+#include <unistd.h>
 #include <getopt.h>
 #include <time.h>
 #include <signal.h>