Workaround for missing std::source_location in apple-clang

On macOSs apple-clang, libc++ may not include std::source_location or std::experimental::source_location. This MR adds a simple implementation (borrowed from the standard library) that is used if no headers are found. Fixes #249 (closed)

GaudiPluginService/include/Gaudi/PluginServiceV2.h

@@ -15,6 +15,7 @@
 /// See @ref GaudiPluginService-readme
 
 #include <Gaudi/Details/PluginServiceDetailsV2.h>
+#include <cstdint>
 #include <functional>
 #include <memory>
 #include <sstream>
@@ -29,11 +30,45 @@
 namespace Gaudi::PluginService::Details {
   using std::source_location;
 }
-#elif __cplusplus >= 201402L
+
+#elif __cplusplus >= 201402L && !defined( __clang__ ) && __GNUC__ >= 8
 #  include <experimental/source_location>
 namespace Gaudi::PluginService::Details {
   using std::experimental::source_location;
 }
+#else
+namespace Gaudi::PluginService::Details {
+
+  struct source_location {
+
+    // 14.1.2, source_location creation
+    static constexpr source_location current( const char* __file = __builtin_FILE(),
+                                              const char* __func = __builtin_FUNCTION(), int __line = __builtin_LINE(),
+                                              int __col = 0 ) noexcept {
+      source_location __loc;
+      __loc._M_file = __file;
+      __loc._M_func = __func;
+      __loc._M_line = __line;
+      __loc._M_col  = __col;
+      return __loc;
+    }
+
+    constexpr source_location() noexcept : _M_file( "unknown" ), _M_func( _M_file ), _M_line( 0 ), _M_col( 0 ) {}
+
+    // 14.1.3, source_location field access
+    constexpr uint_least32_t line() const noexcept { return _M_line; }
+    constexpr uint_least32_t column() const noexcept { return _M_col; }
+    constexpr const char*    file_name() const noexcept { return _M_file; }
+    constexpr const char*    function_name() const noexcept { return _M_func; }
+
+  private:
+    const char*    _M_file;
+    const char*    _M_func;
+    uint_least32_t _M_line;
+    uint_least32_t _M_col;
+  };
+
+} // namespace Gaudi::PluginService::Details
 #endif
 
 namespace Gaudi {