Partial CMakeLists to compile the real icing C++ lib.

Limitations:
* Currently using execute_process instead of add_custom_command, which
  causes protobuf compilation to occur during configure. We need to
  build protoc on the host, so trying to use add_custom_command to do it
  during the target build is complicated.
* Invoking protoc directly instead of using FindProtobuf and
  protobuf_generate_cpp. I was able to get FindProtobuf to work with the
  external/protobuf source tree, but I can't get protobuf_generate_cpp
  to put the generated protos in the right place.
* Icing lib is missing utf dependencies, so this does not currently
  compile.
* No work has been done on the link step; we'll have to find a way to
  compile libprotobuf-lite for the target and depend on it.

Test: ./gradlew icing:assemble
Bug: 149853706
Change-Id: I0568efe23f255b1fca463891ebd816aca5979841
diff --git a/.gitignore b/.gitignore
index e69de29..f57bd5e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -0,0 +1,5 @@
+# Folders
+.cxx/
+
+# Files
+*.iml