Add default reporter for Bazel integration (#2399)

When the added Bazel configuration flag is enabled,
a default JUnit reporter will be added if the XML
envrioment variable is defined.
Fix include paths for generated config header.
Enable Bazel config by default when building with
Bazel.


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

tests/ExtraTests/X30-BazelReporter.cpp

@@ -0,0 +1,17 @@
+
+//              Copyright Catch2 Authors
+// Distributed under the Boost Software License, Version 1.0.
+//   (See accompanying file LICENSE_1_0.txt or copy at
+//        https://www.boost.org/LICENSE_1_0.txt)
+
+// SPDX-License-Identifier: BSL-1.0
+
+/**\file
+ * Test the Bazel report functionality with a simple set
+ * of dummy test cases.
+ */
+
+#include <catch2/catch_test_macros.hpp>
+
+TEST_CASE( "Passing test case" ) { REQUIRE( 1 == 1 ); }
+TEST_CASE( "Failing test case" ) { REQUIRE( 2 == 1 ); }