[3.x] buildsystem: only use search.h detection hack before Android 21

Some functions were added in API 16:

void* tdelete(const void* __key, void** __root_ptr, int (*__comparator)(const void*, const void*)) __INTRODUCED_IN(16);
void tdestroy(void* __root, void (*__free_fn)(void*)) __INTRODUCED_IN(16);
void* tfind(const void* __key, void* const* __root_ptr, int (*__comparator)(const void*, const void*)) __INTRODUCED_IN(16);
void* tsearch(const void* __key, void** __root_ptr, int (*__comparator)(const void*, const void*)) __INTRODUCED_IN(16);

But twalk was added in API 21:

void twalk(const void* __root, void (*__visitor)(const void*, VISIT, int)) __INTRODUCED_IN(21);

At least the VLC implementation of twalk uses the internals of tfind.