apr's configure script uses lots of AC_TRY_RUNs and when cross-compiling
needs a leg-up. This commit adds more configure variables (and removes
one, too).

Notable changes:

 - apr_cv_use_lfs64=yes is removed (again) after having a discussion
   about it with the musl developers. The conclusion was that
   _LARGEFILE64_SOURCE is a horrible thing that we don't want and should
   avoid (hence the removal of the variable, because defining
   _LARGEFILE64_SOURCE is all it does).
 - ap_cv_atomic_builtins is set to "yes" for 64-bit platforms. If
   anybody is interested in enabling this for other targets feel free to
   send a patch after testing it.
 - configure doesn't add -lpthread to LIBS when cross-compiling. This is
   not a problem for musl but might be for other libcs. This commit adds
   to related variable.
 - configure caches are added (via patches) for strerror and /dev/zero
   mmap tests. The former fixes a warning on musl (where strerror
   returns int instead of a pointer) and the latter is required for
   pthread pshared mutex apr_lock implementation and cannot be detected
   during cross-compile either.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>