Bundled catch doesn't bulid with gcc 11

[mattgodbolt]

In file included from /usr/include/signal.h:328,
                 from /home/matthew/dev/zindex/tests/catch.hpp:6442,
                 from /home/matthew/dev/zindex/tests/test_main.cpp:2:
/home/matthew/dev/zindex/tests/catch.hpp:6465:33: error: size of array ‘altStackMem’ is not an integral constant-expression
 6465 |         static char altStackMem[SIGSTKSZ];
      |                                 ^~~~~~~~
/home/matthew/dev/zindex/tests/catch.hpp:6516:45: error: size of array ‘altStackMem’ is not an integral constant-expression
 6516 |     char FatalConditionHandler::altStackMem[SIGSTKSZ] = {};
      |                                             ^~~~~~~~

Going with find_package/conan might help see #40