Defined the implementations of the cpr_*_t opaque structures.
Defined cpr_*_sizeof constants for all opaque structure types.
Ensured that ./configure requires a C++11 compiler ($CXX -std=c++11).

Imported the AX_CXX_COMPILE_STDCXX_11 macro from the Autoconf Archive.

Source: http://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx_11.html