Define HAVE_UNISTD_H on Darwin/macOS: Fixes a compiler failure due to missing function prototype.

configure

@@ -6310,6 +6310,7 @@ case "$host" in
        LIBS="$LIBS -framework OpenGL"
        LIBS="$LIBS -framework OpenAL"
        build_macosx="yes"
+       CFLAGS="$CFLAGS -DHAVE_UNISTD_H"
        ;;
     *-*-cygwin* | *-*-mingw32*)
        { printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking for main in -lopengl32" >&5

configure.ac

@@ -140,6 +140,7 @@ case "$host" in
        LIBS="$LIBS -framework OpenGL"
        LIBS="$LIBS -framework OpenAL"
        build_macosx="yes"
+       CFLAGS="$CFLAGS -DHAVE_UNISTD_H"
        ;;
     *-*-cygwin* | *-*-mingw32*)
        AC_CHECK_LIB([opengl32], [main], [], \

src/slirp/slirp.h

@@ -80,6 +80,10 @@ typedef char *SLIRPcaddr_t;
 # include <stdint.h>
 #endif
 
+#ifdef HAVE_UNISTD_H
+# include <unistd.h>
+#endif
+
 #ifndef _MSC_VER
 #include <sys/time.h>
 #else