tests: cater to systems installing libs in /lib64

tests: cater to systems installing libs in /lib64

See automake bug#11302.

On at least OpenSUSE 12.1, "make install" will install libraries in
the '${prefix}/lib64'  directory by default.  This is due to the
$CONFIG_SITE set by /etc/profile.d/site.sh, which sets a libdir
that ends in '/lib64' rather than '/lib' if it finds that the compiler
is generating 64-bit code.  This behaviour was causing some spurious
failures in our testsuite.  Fix them.

* t/posixsubst-libraries.sh: Assume the libraries are installed in
'$(libdir)' rather than in '$(prefix)/lib'.
* t/posixsubst-ltlibraries.sh: Likewise.
* t/posixsubst-scripts.sh: Similarly, don't assume that '$(bindir)',
'$(sbindir)' and '$(libexedir)' always defaults to respectively
'$(prefix)/bin', '$(prefix)/sbin' and '$(prefix)/libexec'.
* t/transform3.test: Likewise.

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