Improve detection of std::uncaught_exceptions support

The problem was that Catch2 did not reliably include `<exception>`
before it checked for the feature test macro for
`std::uncaught_exceptions`. To avoid overhead of including
`<exception>` everywhere, the configuration check was split out
into a separate header.

Closes #2021

src/CMakeLists.txt

@@ -50,6 +50,7 @@ set(INTERNAL_HEADERS
     ${SOURCES_DIR}/internal/catch_common.hpp
     ${SOURCES_DIR}/internal/catch_compiler_capabilities.hpp
     ${SOURCES_DIR}/catch_config.hpp
+    ${SOURCES_DIR}/internal/catch_config_uncaught_exceptions.hpp
     ${SOURCES_DIR}/internal/catch_console_colour.hpp
     ${SOURCES_DIR}/internal/catch_context.hpp
     ${SOURCES_DIR}/internal/catch_debug_console.hpp