Fixes #3059 by re-applying 08b1c718 from #3087.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=189Signed-off-by: Mario J. Rugiero <mrugiero@gmail.com>

<: is a digraph, so a space is necessary for compilation on certain compilers.

descriptor.proto uses proto2 syntax, which is not ready for external
usage. However, some proto3 files import descriptor.proto and cannot be
used. In this PR, all references (We cheated by only removing
extensions, which is enough for now. User should avoid using messages
defined in descriptor.proto as field type.) to content in
descriptor.proto are removed from generated files. Those that import
descriptor.proto can be used like other proto files.

Fix the bug by sharing the code for generating class name for both
message and enum.

* Use PyUnicode_AsEncodedString() instead of PyUnicode_AsEncodedObject()

* Cherrypick the fix descriptor_pool.FindFileContainingSymbol by extensions.

See https://github.com/google/protobuf/issues/2425

* Issue #2211: Addressing GCC warning on enumeral/non-enumeral in conditional expression.

* Updated per 80 character wrap.

arm-nacl-clang seems to produce a mysterious compiler error when it
encounters __builtin_clzll, so when we are building for NaCl, let's
avoid that in favor of the portable implementation of
Log2FloorNonZero64.

This option will be prepended to generated classes of all messages in
the containing file.

For more info see: https://github.com/google/protobuf/issues/1289

It turns out system headers included by generated plugin.pb.h file already contains
major/minor macro definitions when built on FreeBSD and we need to add #undefs to
the generated header file.

This change also fixes another compile error regarding EXPECT_DEATH on FreeBSD.