Clang complains about "forward declaration of template entity" of all basic_data<>'s members that do not have definitions in `format.h`
```
C:/Users/User/AppData/Roaming/fmt-master/include\fmt/format.h:956:25: warning: instantiation of variable 'fmt::detail::basic_data<>::powers_of_10_64' required here, but no definition is available [-Wundefined-var-template]
static const uint64_t powers_of_10_64[];
^
C:/Users/User/AppData/Roaming/fmt-master/include\fmt/format.h:956:25: note: forward declaration of template entity is here
C:/Users/User/AppData/Roaming/fmt-master/include\fmt/format.h:956:25: note: add an explicit instantiation declaration to suppress this warning if 'fmt::detail::basic_data<>::powers_of_10_64' is explicitly instantiated in another translation unit
```
* add FMT_INSTANTIATION_DEF_API for msvc
This should fix https://github.com/fmtlib/fmt/issues/2228
To fix difference dllexport requirements
msvc: dllexport at template instantiation definition in format.cc
clang: dllexport at template instantiation declaration (extern template) in format.h
* find-package-test fixed by passing CXX_FLAGS, i.e. -stdlib=libc++
* std::array usage in compile-test.cc replaced with plain array, because
<array> header was not included
user-defined constructor added to prevent Intel compilers
warnings. According to the standard, objects need to have a
user-defined constructor if instances are delcared const.
- Fix C++17 Visual Studio builds.
- Use C++17 for `windows-2019` builds.
- Removed options for unsupported compiler (MSVC11).
- Removed options, not needed after enabling C++11+ features in gmock/gtest.
Adding default case for switch statements where the compilation
flag -Wswitch-default is present on the command line when spdlog
is included in external projects.
Signed-off-by: Ryan Sherlock <ryan.m.sherlock@gmail.com>
