In Termux these are implemented in the libutil package to support android-21.

diff -u -r /home/fornwall/lib/android-ndk/sysroot/usr/include/pty.h ./usr/include/pty.h
--- /home/fornwall/lib/android-ndk/sysroot/usr/include/pty.h	2017-06-20 17:41:56.000000000 +0200
+++ ./usr/include/pty.h	2017-07-02 11:42:39.244882732 +0200
@@ -37,10 +37,8 @@
 __BEGIN_DECLS
 
 
-#if __ANDROID_API__ >= 23
-int openpty(int*, int*, char*, const struct termios*, const struct winsize*) __INTRODUCED_IN(23);
-int forkpty(int*, char*, const struct termios*, const struct winsize*) __INTRODUCED_IN(23);
-#endif /* __ANDROID_API__ >= 23 */
+int openpty(int*, int*, char*, const struct termios*, const struct winsize*);
+int forkpty(int*, char*, const struct termios*, const struct winsize*);
 
 
 __END_DECLS