Minor typo in cxx.md

Split libcapnp into three libraries:  libkj, libcapnp, and libcapnpc.  The third contains the schema parser.  Unfortunately it is rather enormous so we should give people the option to not link it.

Fix typo

Use tar-ustar to avoid problems with a long path in the gtest directory that crosses the 99-char tar-v7 limit if the package version number happens to be long enough.  This was causing make distcheck to fail on release candidates, although it worked fine for final releases.

Go back to prefering -std=gnu++11, but detect the one known case where this breaks and we need to use -std=c++11.

Switch to the new compiler.  All tests pass\!  Need to submit bootstrap copies of the generated code used by the compiler itself.  These bootstrap copies are still built by the old compiler at the moment.

Pull TextBlob out of capnpc-capnp and into KJ as kj::StringTree.  Use it in the stringification code to make that code cleaner and avoid using iostreams.

Arrange for compiler to send to the code generator the complete contents of all files listed on the command line, all first-level dependencies of those files, and parent scopes of those dependencies, but nothing more.  This means it's sometimes possible to compile a schema file without parsing all transitive dependencies.  Also, in 'decode' mode, don't compile annotations, so that the files defining those annotations need not be opened at all.