This reverts commit c778945806.
I believe this is exactly what brings the staging branch into
the right shape after the last merge from master (through staging-next);
otherwise part of staging changes would be lost
(due to being already reachable from master but reverted).
Gamin fails to build cross-platform because it tries to run
AC_RUN_IFELSE during configure which fails as the test program is built
for the wrong platform and cannot execute. Since the test is only for
the 'abstract socket namespace' feature, we can just pin the results
for out builds, it is 'yes' for Linux and 'no' for Darwin (and other
BSD).