"make distclean" fails.

"make distclean" fails as some make cannot handle macros that end in '' which is currently the case for SUBDIR in bin/tests if @PKCS11_TOOLS@ is empty. The last element gets repeated which causes the system directory to be entered twice. This can also cause timing issues when building.

-SUBDIRS =      atomic db dst master mem hashes names \
-               net rbt resolver sockaddr tasks timers system \
-               @PKCS11_TOOLS@
+SUBDIR =       atomic db dst master mem hashes names net rbt resolver \
+               sockaddr tasks timers system @PKCS11_TOOLS@