Use pkg-config to find ODBC libraries

[bernhardschmidt]

This was submitted to Debian in https://bugs.debian.org/942412 .
This makes the code substantially easier and might fix ODBC-268.

Using this will probably obsolete #51 and #52 as well.

[bernhardschmidt]

Looks like your Travis Buildroot is too old, the pkgconfig files for odbc where introduced in 2.3.6-0.1, while Ubuntu Xenial has 2.3.4.

[lawrinn]

I can't accept this PR in its current form. First - even your comment suggests, that it won't build on some/older systems. Second - it removes opportunity to build using custom build of the driver manager pointed by parameters/environment, and 3rd - it should not fail if odbc pkgconfig files are not present, but fallback to the old method.
I think I can even remove 2nd item, as odbc_config can be still found in the DM_* directories.

[lawrinn]

One more important thing - we need you write in the comment, that you are contributing your new code of the whole pull request, including one or several files that are either new files or modified ones, under the BSD-new license. But you mentioned somewhere, that you are not the author of the patch ...

[bernhardschmidt]

I have pushed another merge request in #55 that is more in-line wih the current setup. Should just work out of the box, and it has been authored by me. If this is acceptable I can sign-off on the patch.