Add option to force usage of inline namespaces

Detection of inline namespaces is imperfect as some compilers
don't provide __has_feature

This option allows to override it if needed.

include/fmt/core.h

@@ -178,9 +178,17 @@
 #  endif
 #endif
 
-#ifndef FMT_BEGIN_NAMESPACE
+#ifndef FMT_USE_INLINE_NAMESPACES
 #  if FMT_HAS_FEATURE(cxx_inline_namespaces) || FMT_GCC_VERSION >= 404 || \
       (FMT_MSC_VER >= 1900 && !_MANAGED)
+#    define FMT_USE_INLINE_NAMESPACES 1
+#  else
+#    define FMT_USE_INLINE_NAMESPACES 0
+#  endif
+#endif
+
+#ifndef FMT_BEGIN_NAMESPACE
+#  if FMT_USE_INLINE_NAMESPACES
 #    define FMT_INLINE_NAMESPACE inline namespace
 #    define FMT_END_NAMESPACE \
       }                       \