Since they intend to omit gcc, it's more appropriate to simply detect
if there is NDK's clang on PATH, as opposite to requiring to specify it
with CC=clang (and looking for it on PATH).

Also detect NDK version and default to armv7-a for NDK>16.

Address failure to recognize -D__ADNDROID_API__=N in CPPFLAGS.
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5613)