Use isatty() when using GNU libc

While isatty() is a POSIX interface and theoretically could be used
more broadly than on Linux and macOS, use a conservative approach and
use it on any platform that uses GNU libc.

src/catch2/internal/catch_console_colour.cpp

@@ -161,7 +161,7 @@ namespace {
 #endif // Windows/ ANSI/ None
 
 
-#if defined( CATCH_PLATFORM_LINUX ) || defined( CATCH_PLATFORM_MAC )
+#if defined( CATCH_PLATFORM_LINUX ) || defined( CATCH_PLATFORM_MAC ) || defined( __GLIBC__ )
 #    define CATCH_INTERNAL_HAS_ISATTY
 #    include <unistd.h>
 #endif