[c++] A note on test scripts and VPATH builds.

[Alan Conway <ac...@redhat.com>]

Our automake setups allows you to have build output go into the source
tree or to a separate build tree (also known as a VPATH build). It's
useful for having e.g. debug and optimized builds from the same source.

This requires a little care in writing test scripts that work in both
modes. You need to be aware of the difference between the source
directory and the build directory - they're the same for an in-source
build but different for a VPATH build. 

The build directory is always the current directory when the script is
run by make. The source directory can be located as `dirname $0`, i.e.
the directory containing the script itself.
 - temporary files created by the test should be under current dir.
 - references to build output files, e.g. test executables, should be
relative to the current directory.
 - references to source files should be relative to the source directory
`dirname $0` 

I just fixed run_acl_tests to work with VPATH builds, check it out as an
example. 

Cheers,
Alan.