rtlib/android: fix compiles with NDK r15+, in which fseeko/ftello dis…

…appear when targetting API < 24

src/rtlib/unix/fb_unix.h

@@ -21,6 +21,24 @@
 #endif
 typedef off_t fb_off_t;
 
+#if defined HOST_ANDROID
+// __ANDROID_API__ is always defined in newer NDKs, older ones need this header
+#include <android/api-level.h>
+// Defining _FILE_OFFSET_BITS=64 used to be a no-op until NDK r15. 64-bit offset support in 32-bit targets
+// was added to various libc functions over many Android releases, stdio.h mostly in API 24 (Android 7.0).
+// See https://android.googlesource.com/platform/bionic/+/refs/heads/main/docs/32-bit-abi.md
+// (but note it claims ftello/fseeko didn't exist before API 24, actually they did in older NDKs.)
+// On 32-bit targets:
+// Older NDK declare ftello/fseeko, and off_t always as a long so ftello/fseeko are 32-bit.
+// Newer NDK declare 32-bit ftello/fseeko if _FILE_OFFSET_BITS != 64,
+//  64-bit ftello/fseeko if _FILE_OFFSET_BITS == 64 &&  __ANDROID_API__ >= 24,
+//  and neither if _FILE_OFFSET_BITS == 64 &&  __ANDROID_API__ < 24.
+#if __ANDROID_API__ < 24
+#define fseeko(stream, offset, whence) fseek(stream, offset, whence)
+#define ftello(stream)                 ftell(stream)
+#endif
+#endif
+
 FBCALL void fb_BgLock( void );
 FBCALL void fb_BgUnlock( void );
 #define BG_LOCK()   fb_BgLock()