Added _build* pattern to .gitignore.

There is build in top-level directory of the source tree, so we can't
ignore build globally. To make in-source build subfolders usable without
annoying Git prompts about unstaged changes, we can use _build and its
variants (_build-{branch}-{build type}, etc.). It will be ignored.
[SQLite3] Report test6() as skipped
Merge branch 'master' of github.com:SOCI/soci