MDC ignores SPDLOG_NO_TLS

[GaryOderNichts]

The in version 1.14.0 added Mapped Diagnostic Context (MDC) support ignores the SPDLOG_NO_TLS define and always creates a thread_local variable in mdc::get_context.

spdlog/include/spdlog/mdc.h

Lines 40 to 43 in c3aed4b

	static mdc_map_t &get_context() {
	static thread_local mdc_map_t context;
	return context;
	}

This breaks support for systems where thread local storage isn't available.

[AntoineMurat]

I confirm the issue; we are also affected. While it could be mitigated with a (slow) workaround that would use the tid, I believe a reasonable hotfix would be to disable MDC entirely when SPDLOG_NO_TLS is set.

[gabime]

I agree. PR is welcome