[ng] tests: fix spurious failures on NetBSD

[ng] tests: fix spurious failures on NetBSD

* t/built-sources.sh (Makefile.am): In a recipe, use 'printf', not 'echo',
to print a string containing a "\n" sequence, because that is interpreted
like a newline by the echo built-in of NetBSD 5.1 /bin/ksh.
* t/maken.sh: Don't expect the timestamp of the current directory to be
unchanged after "make -n" is run in it; this doesn't hold on NetBSD 5.1.
Instead, make the current directory unwritable before running "make -n",
to ensure attempts to modify the directory fail.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>