the transition from v3 to v4 introduced an API change that format.h does
not provide printf functionality. to easi the transition we introduce a
define `FMT_FORMAT_PROVIDE_PRINTF` that will pull in printf.h from format.h

Added support for pre-c++17 experimental string_view

CMake's GNUInstallDirs knows where particular Linux architectures and
distributions want to have their libraries installed. In particular,
Debian-derived "multi-arch" distributions keep their libraries in triplet
sudirectories under /lib. Other "bi-arch" distributions keep 64-bit
libraries in /lib64.

Including GNUInstallDirs and using CMAKE_INSTALL_LIBDIR and
CMAKE_INSTALL_INCLUDEDIR means that fmt's libraries and header files are
installed in the correct locations.

Tested with OpenEmbedded and on Debian GNU/Linux 9 (the special naming only
applies when installing in /usr.)

FMT_VARIADIC_CONST - Support for const variadic methods

This is to avoid using functionality deprecated in C++17.

./fmt/fmt/format.h(308,10): warning : unknown pragma ignored [-Wunknown-pragmas]
         ^
1 warning generated.
format.cc In file included from fmt\fmt\format.cc:28:
fmt\fmt/format.h(308,10): warning : unknown pragma ignored [-Wunknown-pragmas]
         ^
fmt\fmt\format.cc(165,17): warning : 'strerror' is deprecated: This
function or variable may be unsafe. Consider using strerror_s instead.
To disable deprecation, use _CRT_SECURE_NO_WARNINGS. See online help for
details. [-Wdeprecated-declarations]
      buffer_ = strerror(error_code_);
                ^
C:\Program Files (x86)\Windows Kits\10\Include\10.0.10240.0\ucrt\string.h(178,24) :  note: 'strerror' has been explicitly marked deprecated here
_ACRTIMP char* __cdecl strerror(
                       ^
fmt\fmt\format.cc(78,37): warning : unused function 'strerror_s' [-Wunused-function]
static inline fmt::internal::Null<> strerror_s(char *, std::size_t, ...) {
                                    ^
3 warnings generated.

refactor: use attribute to remove -Wunused-function warnings instead of dummy function call

Also, driveby adding "override" to classes with inherited virtual destructors.

Tests for C++17 std::string_view

Make ArgMap::init not explicitly instantiated

This will remove one more warning that can't be stopped from a calling CMakeLists. This possibly should be set to new at some point (CMake 3.3 behavior would be different / better), as well as 0048.