build: fix "make distcheck" failure

build: fix "make distcheck" failure

Ever since "make distcheck" began using sudo (i.e., running "make check"
as root, and then running it as non-root), the build-after-root would
often fail due to some derived files being owned by root.
This change adds a "chown -R $USER ." to restore order.
* Makefile.am (root-check): Run "chown -R $USER ." after sudo run.
(root_check): Rename variable from root-check, to avoid a warning
from automake about non-POSIX variable.