Add ${API_SOURCES} to `nvim-test` cmake target.

Fixes #720
Don't allow undefined references under Linux.

Many other systems expect this already, but on Linux the default is to
allow them.  Let's turn that off.
Tell cmake to use `-isystem` third-party includes

This adds the `SYSTEM` parameter to `include_directories`, which will tell cmake
to use `-isystem` instead of `-I` for specifying include directories. One
advantage is that compilers won't emit warnings for included files that belong
to dependencies.
Update localization files: Fix build: Fix dangling references.

Problem:  Makefile references some files that don't exist anymore.
Solution: Remove those references.
Update localization files: Fix build: Fix sed issue on OSX.

Problem:  On OSX, sed commands processing files converted to encodings
          other that UTF-8 fail with "RE error: illegal byte sequence".
Solution: Make sed execute with C locale throgh environment variables
          (LANG=C, LC_ALL=C, LC_CTYPE=C).
Update localization files: Fix build: Fix sjis-fixing executable.

Problem:  Executable used to fix japanese files with sjis encoding
          (sjiscorr) fails to compile.
Solution: - Add mising includes.
          - Remove __END_DECLS.
          - Add removed comments.
Update localization files: Fix po files.

Problem:  Build fails because of some messages being repeated.
          Curiously, all repeated messages have this comment:
          "Explicit typecast avoids warning on Mac OS X 10.6".
          No idea why.
Solution: Remove repeated messages.
Update localization files: Update po files.

Sync po files with current code.
Synced to match f0f4ab55002bc9a024b36a6d2c3472ae8583500a.