android: Fix build compatibility with NDK platform 21 and up.

In platforms 21 and higher of the NDK, linux/serial.h is available,
which it was not before. This broke the build, because the configure
script would detect the availability of 'struct serial_struct' in that
header and set HAVE_STRUCT_SERIAL_STRUCT, but the #ifndef __ANDROID__
in libserialport_internal.h stopped us actually including the header.

This change fixes things to build with all versions of the NDK, and is
tested with builds for arm from versions 9 to 24.

Version 21 also added availability of tcdrain(), so we also use that
where available, and only use the direct ioctl() method on NDK < 21.

Fixes #1078.

libserialport_internal.h

@@ -68,8 +68,9 @@
 #endif
 #ifdef __linux__
 #include <dirent.h>
-#ifndef __ANDROID__
-#include "linux/serial.h"
+/* Android only has linux/serial.h from platform 21 onwards. */
+#if !(defined(__ANDROID__) && (__ANDROID_API__ < 21))
+#include <linux/serial.h>
 #endif
 #include "linux_termios.h"
 

serialport.c

@@ -742,7 +742,9 @@ SP_API enum sp_return sp_drain(struct sp_port *port)
 #else
 	int result;
 	while (1) {
-#ifdef __ANDROID__
+#if defined(__ANDROID__) && (__ANDROID_API__ < 21)
+		/* Android only has tcdrain from platform 21 onwards.
+		 * On previous API versions, use the ioctl directly. */
 		int arg = 1;
 		result = ioctl(port->fd, TCSBRK, &arg);
 #else