Use memcpy instead of mempcpy

Summary: Non-GNU platforms don't support mempcpy. As the return value isn't being used
here, memcpy can safely be used instead.
Closes https://github.com/facebook/hhvm/pull/3572

Reviewed By: @sgolemon

Differential Revision: D1517085
Fix casting to wrong type.

Summary: hhvm/hphp/runtime/ext/gd/libgd/gd_webp.cpp:75:10: error: cast from pointer to smaller type 'unsigned int' loses information
                temp = (unsigned uint8*) gdRealloc(filedata, size+GD_WEBP_ALLOC_STEP);
                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Closes https://github.com/facebook/hhvm/pull/3573

Reviewed By: @sgolemon

Differential Revision: D1517087
Use pkg-config to find libmemcached

Summary: libmemcached-0.28 requires linking with an additional library (-lmemcachedutil).  Simplify the cmake code and pick up that additional dependency at the same time by using pkg-config.
Closes https://github.com/facebook/hhvm/pull/3591

Reviewed By: @sgolemon

Differential Revision: D1517441
Rename some things from unserialization-OOM commit

Summary: Names are nice.

Reviewed By: @jasone

Differential Revision: D1517664