__ANDROID__ is always set by the toolchain and less likely to cause
conflicts than ANDROID. This change also removes the -DANDROID flag
we set ourselves in our toolchain CMake files since we don't need
it anymore.
This is a dependency of our upcoming web-based material debugger. This
CL also includes `tnt/CMakeLists.txt`, which builds a static lib in a
minimal configuration that enables WebSocket support. The entire library
is built from only 4 files:
${PUBLIC_HDR_DIR}/CivetServer.h
${PUBLIC_HDR_DIR}/civetweb.h
${SRC_DIR}/civetweb.c
${SRC_DIR}/CivetServer.cpp
