Use a HAVE_MKSTEMP configure check instead of PLATFORM_POSIX.

configure.ac

 AC_CHECK_FUNCS([memmove])
 AC_CHECK_FUNCS([memset])
 AC_CHECK_FUNCS([mkdir])
+AC_CHECK_FUNCS([mkstemp])
 AC_CHECK_FUNCS([pow])
 AC_CHECK_FUNCS([setlocale])
 AC_CHECK_FUNCS([sqrt])

src/libespeak-ng/compiledata.c

 
 		failed = 0;
 
-#ifdef PLATFORM_POSIX
+#ifdef HAVE_MKSTEMP
 		strcpy(fname_temp, "/tmp/espeakXXXXXX");
 		if ((fd_temp = mkstemp(fname_temp)) >= 0)
 			close(fd_temp);

src/libespeak-ng/readclause.c

 			fclose(f);
 			f = NULL;
 
-#ifdef PLATFORM_POSIX
+#ifdef HAVE_MKSTEMP
 			strcpy(fname_temp, "/tmp/espeakXXXXXX");
 			if ((fd_temp = mkstemp(fname_temp)) >= 0)
 				close(fd_temp);