Fix build caching (#242)

Summary:
See https://github.com/facebook/flipper/issues/230 for a longer
explanation.

It turns out that many of the tasks in `native.gradle` (at least
`prepareGlog` which modifies files inside the source directory, but also
others) cause the cmake cache to invalidate.

Because we run those tasks unconditionally, we need to recompile all our
C++ code even when unmodified.

I fix this by adding a manual cache key to this which simply skips all
the tasks unless an up-to-date `external_cache_revision.txt` is found.
This is a bit hacky, but the alternatives are a) convincing cmake that
these modifications are not recompile-worthy which I have no idea how to
do and which might be impossible to do `mtime` on the files touches, or
b) having fine-grained `onlyIf` rules for every task which can easily
break. This seems the most straight-forward option.

Fixes #230.
Pull Request resolved: https://github.com/facebook/flipper/pull/242

Reviewed By: jknoxville

Differential Revision: D9381340

Pulled By: passy

fbshipit-source-id: a7db55aacac4a29076c29298d6c5b97d8bc91f66

xplat/CMakeLists.txt

@@ -48,7 +48,6 @@ set(rsocket_build_DIR ${build_DIR}/rsocket/${ANDROID_ABI})
 
 file(MAKE_DIRECTORY ${build_DIR})
 
-#add_subdirectory(${libfolly_DIR} ${libfolly_build_DIR})
 add_subdirectory(${rsocket_DIR} ${rsocket_build_DIR})
 
 message(STATUS "RSocket DIR:- " ${rsocket_DIR})