We follow the findProtobuf.cmake naming convention
to ease the use of protobuf as cmake subproject

Add implementations of open(2), mkdir(2), stat(2),
etc. that support long paths under Windows (paths
longer than MAX_PATH in <windows.h>, which is 260
characters).

The implementations are in a separate namespace
(google::protobuf::internal::win32), so they won't
collide with the standard implementations in
<io.h>, but after importing them with `using` they
can be drop-in replacements.

Fixes https://github.com/bazelbuild/bazel/issues/2634
Fixes https://github.com/google/protobuf/issues/2891

tested only on windows with visual studio 2015 as generator

When trying to compile the protobuf code as a DLL, and then compile other DLLs
with generated .pb.cc/h files that reference
InternalMetadataWithArena::InternalMetadataWithArena(Arena*), MSVC gives an
"unresolved external symbol" error.  This seems to be due to the function being
simultaneously exported and inline.  Moving it out-of-line fixes things.

There are other functions exported and inline as well but de-inlining them
doesn't seem to be necessary to get the build working, and I'd rather de-inline
as few functions as possible.

Change-Id: I17e16fdae6e4d2fb74d178fa5564a609ed58af1d

Change-Id: Ieb7a2c2fbf35bc2a8fa65b915a5ecb68c83863e4

cmake targets that depend on these libraries will automatically have
these include directories.

[ci skip]

Change-Id: I9391c09640e0b0b2b21c45a97a1fc91814d95c5d