android: support api level less than 21
android: support api level less than 21

Remove restriction to Android API level >= 21, now libuv can be used with
Android API level at least 14.

Direct use of getpwuid_r function resulted in linker errors in applications
were built against API level below 21. That function was 'officially'
introduced in Android API level 21, but it exists in libc beginning from API
level at least 14.

So try to get a pointer to getpwuid_r at runtime using dlsym when building
libuv with API level < 21.

PR-URL: https://github.com/libuv/libuv/pull/633
Reviewed-By: Ben Noordhuis <59bd0a3ff43b32849b319e645d4798d8a5d1e889@bnoordhuis.nl>
Reviewed-By: Saúl Ibarra Corretgé <8a7991f6950ea189abf159d6ec98d2fcc13dc5f6@gmail.com>

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