Add cmake support.

[xfxyjwf]

Add cmake support primarily for Visual Studio users. These scripts can also be used to build protobuf on Linux, MinGW, Cygwin as well.

This PR also includes a shell script used to copy file lists from src/Makefile.am to cmake files so we don't need to do that manually.

@pherl to review

[liujisi]

It looks like text after '..' becomes grey for some reason. Please double check if it is some special mark down syntax.

[xfxyjwf]

Done. Fixed the formatting of this file.

[liujisi]

Should we delete the current msvc proj files? Or should we generated them in releases and do not include them in git?

[paleozogt]

I vote for leaving them out of git. The generated project files can be thought of in the same way as binaries-- they're compiled artifacts generated from the source.

[jtattermusch]

I think commiting the generated projects makes sense. It's easier for users to start when they already have their vsprojects ready once they clone - non everyone has cmake installed.

if we commit the vsproject files, we need to make sure they are always up to date (there should be a presubmit check that verifies that regenerating the projects using cmake yields no diff).

[xfxyjwf]

Vsproject files generated by cmake actually depend on cmake. When you build the solution, it will run cmake to check whether any dependencies have changed and regenerate vsproject files if necessary. The last time I checked, they have no plan to update cmake to generate *clean" vsproject files that can work alone without cmake. So users would have to install cmake anyway regardless of whether we check-in the generated files. Given this, checking in these files provides little value and we probably shouldn't do that.

[liujisi]

LGTM