fix variant detection with non-clang compiler

[robinmoussu]

I noticed that stringification support for std::variant isn't activated if your compiler is gcc or clang >= 8. This should correct it.

[robinmoussu]

Using gcc9 (not tested with other compilers), when the C++17 mode is activated, the following test fails:

../projects/SelfTest/UsageTests/ToStringVariant.tests.cpp:37: FAILED:
  REQUIRE( value.valueless_by_exception() )
with expansion:
  false

I don't understand why however.

For reference:

 30     SECTION("valueless by exception") {
 31         struct sample {
 32             operator int() const { throw 42; }
 33         };
 34
 35         type value{1.5f};
 36         REQUIRE_THROWS_AS( value.emplace<int>(sample{}), int );
 37         REQUIRE( value.valueless_by_exception() );
 38         CHECK( "{valueless variant}" == ::Catch::Detail::stringify(value) );
 39     }

[codecov]

Codecov Report

Merging #1511 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1511   +/-   ##
=======================================
  Coverage   80.53%   80.53%           
=======================================
  Files         121      121           
  Lines        3436     3436           
=======================================
  Hits         2767     2767           
  Misses        669      669

[codecov]

Codecov Report

Merging #1511 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1511   +/-   ##
=======================================
  Coverage   80.53%   80.53%           
=======================================
  Files         121      121           
  Lines        3436     3436           
=======================================
  Hits         2767     2767           
  Misses        669      669

[horenmar]

The problem with the test has been solved in a different PR.