spdlog fails to build with latest fmt

[H1X4Dev]

Using latest fmt, the spdlog will fail to build.

You must change the basic_runtime to runtime_format_string to fix the issue, however bundled spdlog uses basic_runtime. Line: https://github.com/gabime/spdlog/blob/v1.x/include/spdlog/common.h#L181

[gabime]

PR is welcome.

[H1X4Dev]

It seems like won't really be an easy fix without hacky ways, such as definining "FMT_VERSION_MASTER" on cmake option, because their Release fmt version is the same as master fmt, but with breaking differences. If this is something allowed to add, I can make the patch for it, otherwise I don't know a valid fix that would be compatible with both versions.

[gabime]

The version in fmt head is [the unreleased) 90101, while spdlog use 90100. We can check the fmt version and apply the fix if it is >= 90101