C++-ify source code

C++-ify source code

The ccache source code will be converted to C++, targeting C++11. This
commit only arranges the existing C-style code to be built as C++ code.
This makes it possible to call new C++ code from old C-style code.
Gradual conversion to C++ functionality and idioms will follow in a slow
and controlled fashion – no big bang rewrites.

The alternative would be to convert code in a top-down fashion, i.e.
only calling legacy C code from new C++ code, not the other way around.
That approach is however not a good idea since the code that will
benefit most from being written in proper C++ is code deep down in the
call chains.

Except for renaming source code files to .cpp and .hpp, this commit
makes minimal changes to make the code base buildable again, for
example:

- Instructs configure.ac to look for a mandatory C++11-compliant
  compiler.
- Adds Makefile rules for building C++ code.
- Sets up Travis-CI to pass C++ compiler flags and similar to the build.
- Adds new casts where needed.
- Adds const keywords where needed.
- Renames variables to something else than C++ keywords (e.g.
  “template”).
- Rearranges some code to avoid complaints about goto jumps that cross
  variable lifetimes.