Adds pkg-config (`.pc`) files for CMake based installs.

This allows the file path to contain spaces which is very common on
windows.

Ruby: generated foo.proto -> foo_pb.rb instead of foo.rb.

This brings us more into line with other langauges, and makes it more
obvious when we are requiring protobuf generated code.

Add a script to build protoc zip packages.

[ci skip]

Change-Id: Ia0e0bf3bb6bd24633dc8f294318bb20ac837f8b9

Ruby: translate package names from snake_case -> PascalCase.

Added unit test for PascalCasing package names in Ruby.

Added new file to ruby_EXTRA_DIST.

A series of improvements:

- Improved Protobuf module compatibility (disabled by default);
- Hide advanced settings;
- Added build tree configuration;
- Added build of examples.

Fix the cmake configuration file install path to be more standards compliant (See the description of cmake's config search behavior on https://cmake.org/cmake/help/v3.4/command/find_package.html)

Additional export as "protobuf" package
for importing from other cmake projects