Basic Alembic support

All in all, this patch adds an Alembic importer, an Alembic exporter,
and a new CacheFile data block which, for now, wraps around an Alembic
archive. This data block is made available through a new modifier ("Mesh
Sequence Cache") as well as a new constraint ("Transform Cache") to
somewhat properly support respectively geometric and transformation data
streaming from alembic caches.

A more in-depth documentation is to be found on the wiki, as well as a
 guide to compile alembic: https://wiki.blender.org/index.php/
User:Kevindietrich/AlembicBasicIo.

Many thanks to everyone involved in this little project, and huge shout
out to "cgstrive" for the thorough testings with Maya, 3ds Max, Houdini
and Realflow as well as @fjuhec, @jensverwiebe and @jasperge for the
custom builds and compile fixes.

Reviewers: sergey, campbellbarton, mont29

Reviewed By: sergey, campbellbarton, mont29

Differential Revision: https://developer.blender.org/D2060

build_files/cmake/config/blender_lite.cmake

@@ -8,6 +8,7 @@
 set(WITH_INSTALL_PORTABLE    ON  CACHE BOOL "" FORCE)
 set(WITH_SYSTEM_GLEW         ON  CACHE BOOL "" FORCE)
 
+set(WITH_ALEMBIC             OFF CACHE BOOL "" FORCE)
 set(WITH_BUILDINFO           OFF CACHE BOOL "" FORCE)
 set(WITH_BULLET              OFF CACHE BOOL "" FORCE)
 set(WITH_CODEC_AVI           OFF CACHE BOOL "" FORCE)