Rename BUILD as BUILD.bazel (#537)

Bazel natively supports build files named BUILD and BUILD.bazel[0].
Using BUILD conflicts with CMake when building on case-insensitive file
systems such as the default filesystem of macOS machines.

[0]: https://docs.bazel.build/versions/master/build-ref.html#packages
diff --git a/BUILD b/BUILD.bazel
similarity index 100%
rename from BUILD
rename to BUILD.bazel