[build] Fixed installation of rootmap files

Installation of rootmap files was broken for out-of-tree builds,
also causing "make distcheck" to fail.
[build] Removed install-exec-hook targets

For some reason, "make distcheck" does not work if with the
install-exec-hook targets in the Makefile.am files - it somehow results
in a read-only distcheck "_inst" directory and distcheck fails.