Strip leading ./ from __FILE__ in __exp__ macro

clang puts a ./ in front of __FILE__, which gcc does not. To remove this
difference, we add the constexpr drop_leading_dot_slash() which removes this
difference between both compilers.

Author: D4N

lib/extensions.hpp

@@ -146,8 +146,12 @@
                               __funcname__, __filename__, __line__}; }
 
 
+constexpr const char * drop_leading_dot_slash(const char * path) {
+  return path[0] == '.' && path[1] == '/' ? path + 2 : path;
+}
+
 // for in-place throw parameter
-#define EXP(TROW_REASON) __exp__(TROW_REASON, __func__, __FILE__, __LINE__)
+#define EXP(TROW_REASON) __exp__(TROW_REASON, __func__, drop_leading_dot_slash(__FILE__), __LINE__)