fixup! boost-lib: enable parallel build

This fixes commit f9d5bde3.

We also often have MAKEFLAGS with -l option. For b2, this option sets
a time limit in seconds for each job, which led to random failures.
3 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

07:14:08

passed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

01:30:21

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

02:44:26