feat: explicit this enabled on clang-18

src/core/include/mp-units/bits/external/hacks.h

@@ -83,6 +83,13 @@
 
 #endif
 
+// TODO revise the below when clang-18 is released
+#if MP_UNITS_COMP_CLANG >= 18 && __cplusplus >= 202300L && !defined __cpp_explicit_this_parameter
+
+#define __cpp_explicit_this_parameter 202110L
+
+#endif
+
 #if (defined MP_UNITS_COMP_CLANG && MP_UNITS_COMP_CLANG < 17) || (defined MP_UNITS_COMP_GCC && MP_UNITS_COMP_GCC < 12)
 
 #define MP_UNITS_CONSTEVAL constexpr