Use std::function for the global error handler

Fix signed/unsigned mismatch in VS

Fix C++20 build resulting in deprecated implicit copy assignment operator warning

Allow compilation with nvc++ (and possibly PGI)

add macros for overriding the individual level names

Revert "added spdlog::level::set_string_view to enable alternate log level names without changing the build via SPDLOG_LEVEL_NAMES"

This reverts commit 2a16d1d2.

Revert "remove constexpr on level_string_views to fix compilation on C++17 from addition of set_string_view"

This reverts commit ac3e26b0.

Add support for CMake policy CMP0077

small std::find conversion

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Fix warning about testing _WIN64 which might be undefined

This warning is disabled by default, but is pretty useful and worth
enabling for MSVC, just as -Wundef for gcc, so fix it in Win32 build.

Avoid harmless warning about unreachable statement in MSVS build

All MSVS versions >= 2015 warn about "return 0" after throw_spdlog_ex()
being unreachable in filesize(), so disable this warning in this
function (note that it can't be disabled inside it).

Change c-style casts to reinterpret casts in tcp_client

Made mutex member variable mutable

Classes inheriting from `base_sink` can now lock the base mutex inside their `const` member methods (e.g., basic accessors).

fix Xcode compiler warning "no new line at the end of file"