Simplify feature-detection for std::source_location

include/mettle/detail/source_location.hpp

@@ -1,37 +1,34 @@
 #ifndef INC_METTLE_DETAIL_SOURCE_LOCATION_HPP
 #define INC_METTLE_DETAIL_SOURCE_LOCATION_HPP
 
-#include <ciso646>
+#include <version>
 
 // Try to use `std::source_location`, except on Clang 15, where it's broken.
 // See <https://github.com/llvm/llvm-project/issues/56379>.
-#if __has_include(<source_location>) &&                 \
+#if defined(__cpp_lib_source_location) &&               \
+    __has_include(<source_location>) &&                 \
     (!defined(__clang__) || __clang_major__ >= 16)
 #  include <source_location>
-#  ifdef __cpp_lib_source_location
-#    define METTLE_FOUND_SOURCE_LOCATION
+
 namespace mettle::detail {
   using source_location = std::source_location;
 }
-#  endif
-#endif
 
-#if !defined(METTLE_FOUND_SOURCE_LOCATION) &&           \
-    __has_include(<experimental/source_location>)
+#elif !defined(METTLE_FOUND_SOURCE_LOCATION) &&         \
+      __has_include(<experimental/source_location>)
 #  include <experimental/source_location>
-#  define METTLE_FOUND_SOURCE_LOCATION
+
 namespace mettle::detail {
   using source_location = std::experimental::source_location;
 }
-#endif
 
-#ifndef METTLE_FOUND_SOURCE_LOCATION
+#else
 #  include "source_location_shim.hpp"
+
 namespace mettle::detail {
   using source_location = source_location_shim;
 }
-#endif
 
-#undef METTLE_FOUND_SOURCE_LOCATION
+#endif
 
 #endif