C++20 fixes

Fix compilation with gcc13 / c++20 , ROOT 6.28.04 (LCG dev4), fmtlib 10.0.0

• use fmt::runtime where needed (with a hack to still work with fmtlib < 8)
• avoid implicit capture of this in lambdas via [=]
• fix comparison between Gaudi::Property<std::string> and const char*
• hide std::source_location to cling

Closes #266 (closed)

GaudiPluginService/include/Gaudi/PluginServiceV2.h

@@ -25,18 +25,12 @@
 #include <typeinfo>
 #include <utility>
 
-#if __cplusplus > 201703L && __has_include( <source_location> )
-#  if defined( __clang__ )
-#    include <experimental/source_location>
-namespace Gaudi::PluginService::Details {
-  using std::experimental::source_location;
-}
-#  else
-#    include <source_location>
+#if __cplusplus > 201703L && __has_include( <source_location> ) && !defined (__CLING__)
+#  include <source_location>
 namespace Gaudi::PluginService::Details {
   using std::source_location;
 }
-#  endif
+
 #elif __cplusplus >= 201402L && !defined( __clang__ ) && __GNUC__ >= 8
 #  include <experimental/source_location>
 namespace Gaudi::PluginService::Details {