Fix build issues under C++11

include/spdlog/common.h

@@ -55,9 +55,15 @@
 #if defined(_MSC_VER) && (_MSC_VER < 1900)
 #    define SPDLOG_NOEXCEPT _NOEXCEPT
 #    define SPDLOG_CONSTEXPR
+#    define SPDLOG_CONSTEXPR_FUNC
 #else
 #    define SPDLOG_NOEXCEPT noexcept
 #    define SPDLOG_CONSTEXPR constexpr
+#    if __cplusplus >= 201402L
+#        define SPDLOG_CONSTEXPR_FUNC constexpr
+#    else
+#        define SPDLOG_CONSTEXPR_FUNC
+#    endif
 #endif
 
 #if defined(__GNUC__) || defined(__clang__)

include/spdlog/details/fmt_helper.h

@@ -34,7 +34,7 @@ template<typename T>
 inline void append_int(T n, memory_buf_t &dest)
 {
     // Buffer should be large enough to hold all digits (digits10 + 1) and a sign
-    SPDLOG_CONSTEXPR auto BUF_SIZE = std::numeric_limits<T>::digits10 + 2;
+    SPDLOG_CONSTEXPR const auto BUF_SIZE = std::numeric_limits<T>::digits10 + 2;
     char buf[BUF_SIZE];
 
     auto [ptr, ec] = std::to_chars(buf, buf + BUF_SIZE, n, 10);

include/spdlog/fmt/bin_to_hex.h

@@ -96,7 +96,7 @@ struct formatter<spdlog::details::dump_info<T>, char>
 
     // parse the format string flags
     template<typename ParseContext>
-    SPDLOG_CONSTEXPR auto parse(ParseContext &ctx) -> decltype(ctx.begin())
+    SPDLOG_CONSTEXPR_FUNC auto parse(ParseContext &ctx) -> decltype(ctx.begin())
     {
         auto it = ctx.begin();
         while (it != ctx.end() && *it != '}')