Alter distclean rule to prevent recursing into the same directory twice

Alter distclean rule to prevent recursing into the same directory twice

Commit f87e0c03ee removed the "system" directory from the TESTDIRS
variable in bin/tests/Makefile.in in an attempt to fix "make distclean"
which was broken since commit 0d784de16a.  However, this change
prevented any system tests from being run when "make test" is invoked.

We now put it back into both SUBDIRS and TESTDIRS, but with a modified
rule to check for the existence of a Makefile in each subdirectory before
trying to run make there. This prevents "make distclean" from trying to
run again in a directory where it's already been run.