Compatility fixes for GCC5 (#2448)

* GCC5 compat: work around inherited constructor issues

Don't use inherited constructors, forward manually instead. This
basically reverts 61f803126d.

I believe that GCC5 does not implement P0136, a C++17 change that made
inherited constructors actually usable and was backported as a DR all
the way to C++11.

* GCC5 compat: bypass std::pair construction issue

Co-authored-by: Martin Hořeňovský <martin.horenovsky@gmail.com>

src/catch2/internal/catch_reporter_spec_parser.cpp

@@ -126,7 +126,7 @@ namespace Catch {
                     return {};
                 }
 
-                auto ret = kvPairs.emplace( kv.key, kv.value );
+                auto ret = kvPairs.emplace( std::string(kv.key), std::string(kv.value) );
                 if ( !ret.second ) {
                     // Duplicated key. We might want to handle this differently,
                     // e.g. by overwriting the existing value?