🐛 [ut] Source-file-location fails on macOS, giving unknown, Fix #291

Problem:
- On macOS, with both g++9 homebrew and clang-9 MacPorts the UT tests all fail with `unknown`.

Solution:
- Fix detection of `source location` builtins.
  Use `__has_builtin` for clang based compilers.
  Enable it by default for GCC >= 9 (GCC HEAD has support for it).
  Disable it for MSVC which doesn't support these builtins yet.

include/boost/ut.hpp

@@ -39,6 +39,14 @@ export import std;
 #else
 #define BOOST_UT_VERSION 1'1'6
 
+#if not defined(__has_builtin)
+#if (__GNUC__ >= 9)
+#define __has___builtin_FILE 1
+#define __has___builtin_LINE 1
+#endif
+#define __has_builtin(...) __has_##__VA_ARGS__
+#endif
+
 #if defined(BOOST_UT_FORWARD)
 namespace std {
 template <class TLhs, class TRhs>
@@ -210,7 +218,8 @@ namespace reflection {
 class source_location {
  public:
   [[nodiscard]] static constexpr auto current(
-#if ((__GNUC__ >= 9 or __clang_major__ >= 9) and not defined(__APPLE__))
+
+#if (__has_builtin(__builtin_FILE) and __has_builtin(__builtin_LINE))
       const char* file = __builtin_FILE(), int line = __builtin_LINE()
 #else
       const char* file = "unknown", int line = {}

test/ut/ut.cpp

@@ -1344,7 +1344,7 @@ int main() {
     test_assert("2 == 2" == test_cfg.assertion_calls[1].expr);
   }
 
-#if ((__GNUC__ >= 9 or __clang_major__ >= 9) and not defined(__APPLE__))
+#if (__has_builtin(__builtin_FILE) and __has_builtin(__builtin_LINE))
   {
     test_cfg = fake_cfg{};