On Gentoo Linux (and possibly others?) with Subversion 1.12.0,
compilation of hydra-svn.c fails with:
In file included from /usr/include/subversion-1/svn_client.h:34,
from hydra-svn.c:9:
/usr/include/apr-1/apr.h:632:2: error: #error no decision has been made on APR_PATH_MAX for your platform
#error no decision has been made on APR_PATH_MAX for your platform
^~~~~
This happens when PATH_MAX is not defined.
PATH_MAX is defined by /usr/include/linux/limits.h, but rather than
include'ing that directly and possibly breaking other platforms,
include sys/param.h (which will include linux/limits.h indirectly)
iff PATH_MAX is not defined and sys/param.h exists.
I based the approach on how math.h is handled.
