uv: support android libuv standalone build

Tested most of my compilation in the previous patch on NodeJS
and extracted the patches from there. This patch ensures libuv
will be capable of building standalone as well, both with gyp
and Makefiles.

Build documentation was added to the README.md file.

Some tests are failing, and I have not heavily investigated
the reasons. The failures are generally on errors, and are
likely related to differences between fully POSIX-compatible
systems and android.
build: fix build breakage from 3fdd2a1

Make sure the android_build variable is defined.
src: make queue.h c++ compatible

Squelch a warning about a cast from void* to another pointer type.
It's legal C but not legal C++. Makes queue.h usable in node.js.
unix: s/ngx-queue.h/queue.h/ in checksparse.sh
Corrected android OS handling

Since OS is transformed from android
to linux, we need to ensure that conditions
specifying OS==linux are prevented from
executing by additionally specifying
'and android_build=0'.