Allow custom libexec location

To install for different distributions, LIBEXEC_PREFIX can now be set.
It defaults to $(PREFIX)/usr/lib/bees as used in most common
distributions.

Local overrides are possible by setting variables in a "localconf" file
which will be included by the Makefile if it exists.

For some distributions you may want to set it to /usr/libexec or
/usr/libexec/bees.

.gitignore

@@ -10,3 +10,4 @@ html/
 latex/
 make.log
 make.log.new
+localconf