See https://github.com/android-ndk/ndk/issues/440

diff -u -r /home/fornwall/lib/android-ndk/sysroot/usr/include/string.h ./usr/include/string.h
--- /home/fornwall/lib/android-ndk/sysroot/usr/include/string.h	2017-06-20 17:41:56.000000000 +0200
+++ ./usr/include/string.h	2017-06-30 22:15:01.161342640 +0200
@@ -74,12 +74,12 @@
 #endif /* __ANDROID_API__ >= 18 */
 
 #if defined(__USE_GNU)
+#if __ANDROID_API__ >= 24
 #if defined(__cplusplus)
 extern "C++" char* strchrnul(char* _Nonnull, int) __RENAME(strchrnul) __attribute_pure__;
 extern "C++" const char* strchrnul(const char* _Nonnull, int) __RENAME(strchrnul) __attribute_pure__;
 #else
 
-#if __ANDROID_API__ >= 24
 char* strchrnul(const char* _Nonnull, int) __attribute_pure__ __INTRODUCED_IN(24);
 #endif /* __ANDROID_API__ >= 24 */
 
@@ -169,6 +169,7 @@
 #endif
 
 #if defined(__USE_GNU) && !defined(basename)
+#if __ANDROID_API__ >= 23
 /*
  * glibc has a basename in <string.h> that's different to the POSIX one in <libgen.h>.
  * It doesn't modify its argument, and in C++ it's const-correct.
@@ -178,7 +179,6 @@
 extern "C++" const char* basename(const char* _Nonnull) __RENAME(__gnu_basename);
 #else
 
-#if __ANDROID_API__ >= 23
 char* basename(const char* _Nonnull) __RENAME(__gnu_basename) __INTRODUCED_IN(23);
 #endif /* __ANDROID_API__ >= 23 */