Parametrize parallelization level on the OSX buildbot.

[in tools/buildbot/slaves/svn-x64-macosx]
* setenv.sh: Set a new parameter, SVNBB_PARALLEL.
* svnbuild.sh, svncheck.sh: Use this parameter in the 'make'
   and 'make check' command lines.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1638767 13f79535-47bb-0310-9956-ffa450edef68