Add support for FMT_STRING compile time checking. Add negative compilation unit test for compilers and c++ standard that support relaxed constexpr.

tests/test_compilation_failures.cpp

@@ -0,0 +1,14 @@
+#include "includes.h"
+
+TEST_CASE("{fmt} FMT_STRING functionality preserved (negative test)", "[fmt][fail][fail compilation]")
+{
+    std::ostringstream oss;
+    auto oss_sink = std::make_shared<spdlog::sinks::ostream_sink_mt>(oss);
+
+    spdlog::set_default_logger(std::make_shared<spdlog::logger>("oss", oss_sink));
+    spdlog::default_logger()->set_level(spdlog::level::trace);
+
+    spdlog::info(FMT_STRING("The best part of {{fmt}} is the compile time checking: {:d}"), "I shouldn't compile");
+    // This should never be able to compile, so running is a failure.
+    FAIL("This test case isn't meant to compile, let alone run.");
+}