By hand, or in slackbuild script
copy attached patch file (rename it to .patch) to slackbuild dir, and add this line to You slackbuild just before make invocation:
patch -p0 < $(CWD)/slack_boost.patch
During the execution of slackbuild script You may be warned, that "previous patch applied" - say yes on that question (You may be asked, but also You may be not
it shouldn't ask, and I don't know, why it asked me
If You have problems, PM me - I will send You my slackbuild script.