maint: improve dist-check.mk rules

maint: improve dist-check.mk rules

* dist-check.mk (null_AM_MAKEFLAGS): Remove LIBTOOL.  Adding it was
erroneous, since it is required when building from a distribution
tarball of a libtool-using project.  Reported by Ralf Wildenhues.
(my-distcheck): Reorganize to use a subshell and set -e, so that
failures propagate "out".  Without this change, setting LIBTOOL=false
would cause a failure that would then be ignored, probably due to a
problem in $(install-transform-check).