build: don't use foo_LDADD if there's no foo program

build: don't use foo_LDADD if there's no foo program

For example, instead of factoring out the extra linker flags needed
by 'cp', 'mv' ind 'install' into a '$(copy_LDADD)' variable, factor
them out in a '$(copy_ldadd)' variable.

Partly a minor cleanup, partly a preparation for future changes.

* src/Makefile.am (copy_LDADD): Rename ...
(copy_ldadd): ... like this.
(remove_LDADD): Rename ...
(remove_ldadd): ... like this.
All uses adjusted.  Some comments updated.