Modify the condition of SPDLOG_CONSTEXPR_FUNC to match that of fmt

fix the issue where constexpr function in spdlog may call non-constexpr function in the bundled fmt because FMT_USE_CONSTEXPR is not defined.

include/spdlog/common.h

@@ -65,15 +65,21 @@
 #if defined(_MSC_VER) && (_MSC_VER < 1900)
     #define SPDLOG_NOEXCEPT _NOEXCEPT
     #define SPDLOG_CONSTEXPR
-    #define SPDLOG_CONSTEXPR_FUNC inline
 #else
     #define SPDLOG_NOEXCEPT noexcept
     #define SPDLOG_CONSTEXPR constexpr
-    #if __cplusplus >= 201402L
+#endif
-        #define SPDLOG_CONSTEXPR_FUNC constexpr
+
-    #else
+#ifndef __has_feature
-        #define SPDLOG_CONSTEXPR_FUNC inline
+#    define __has_feature(x) 0
-    #endif
+#endif
+
+#if (__has_feature(cxx_relaxed_constexpr) || (defined(_MSC_VER) && (_MSC_VER >= 1912)) ||                                                  \
+     (defined(__GNUC__) && __GNUC__ >= 6 && defined(__cplusplus) && __cplusplus >= 201402L)) &&                                            \
+    !defined(__ICL) && !defined(__INTEL_COMPILER) && !defined(__NVCC__)
+    #define SPDLOG_CONSTEXPR_FUNC constexpr
+#else
+    #define SPDLOG_CONSTEXPR_FUNC inline
 #endif
 
 #if defined(__GNUC__) || defined(__clang__)