libsmbclient.php: remove broken example file

The example file no longer worked. Rely instead on extensive
documentation and inline examples in README.md.
config.m4: rewrite, search for library and headers

The extension no longer compiled out of the box with Samba-4.x,
because Samba moved its include files from /usr/include to
/usr/include/samba-4.x. Add this new location to the list of search
paths.

Do the shared library lookup in the standard PHP way (gleaned from
several other extensions) to avoid using hardcoded paths and filenames
for the shared library (not portable).

Try to do something sensible with the DIR argument to
--with-libsmbclient=DIR: if not given, the system libraries are used,
else the script expects the library to be in DIR/lib, and the headers in
DIR/include.
README.md: make file-reading example binary safe

The old file-reading example did not correctly check the loop
terminating condition; the correct check is whether the data is false or
the zero string. The old loop would also exit on "falsy" data, such as
the string "0".
Bump version number to 0.5.0