Add C++ bindings

Add C++ bindings

Add argpar.hpp, providing C++ bindings around the argpar public API.

 - Everything is under the `argpar` namespace.
 - Create an argpar argument iterator with the `Iter` constructor:

     Iter(unsigned int argc, const char * const *argv,
	  const argpar_opt_descr_t *descrs);

   For simplicity and to avoid adding unnecessary costs to the
   abstraction, the constructor accepts an array of
   `argpar_opt_descr_t`.
 - Call next() to obtain the next item.  If the end of the command line
   has been reached, an empty optional is returned.
 - Differentiate between opt and non-opt argument items using
   argpar::Item::type(), argpar::Item::isOpt() or
   argpar::Item::isNonOpt().
 - Parse errors are reported using exceptions.  Specific exception types
   derive from `argpar::Error`.

Improve the testsuite to cover the C++ API.  The test program is now
split in 3 files:

 - test-argpar-c.c: provides the test_succeed_c() and test_fail_c()
   functions, testing a given case using the C API.
 - test-argpar-cxx.cpp: provides the testSucceedCxx() and testFailCxx()
   functions, testing a given case using the C++ API.
 - test-argpar.cpp: contains the test cases, calls the C and C++ test
   functions for each case.

Make configure look for a C++11 compiler.  Add the necessary m4 files
from autoconf-archive.

In the C++ test, we need an `optional` type to provide as a template
parameter of `argpar::Iter`.  Given that we compile as C++11, we can't
use `std::optional`.  Add a copy of `optional.hpp` from the
optional-lite project [1].

Add a copy of the {fmt} library [2], in order to format strings
conveniently in the test.

Philippe updated `README.adoc` to mention the new C++ API.

[1] https://github.com/martinmoene/optional-lite
[2] https://github.com/fmtlib/fmt

Change-Id: Iacf4da8837e8dea79b770346f6fcf3406f0668db
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>