libapparmor: check if AX_CHECK_COMPILE_FLAG is available

The error message when autoconf-archive is not installed is not very intuitive:

./configure: line 14422: EXTRA_WARNINGS: command not found
./configure: line 14423: syntax error near unexpected token `-flto-partition=none,'
./configure: line 14423: `AX_CHECK_COMPILE_FLAG(-flto-partition=none, , , -Werror)'

So, check if AX_CHECK_COMPILE_FLAG is defined and if not, complain that autoconf-archive is missing.

Signed-off-by: Georgia Garcia georgia.garcia@canonical.com