make the build in the the build directory to not pollute the source tree

Copied from abrt commit 8efc696f2595805d877e0b48cb47b3d92caf99fa.

Closes #116.

Signed-off-by: Martin Milata <mmilata@redhat.com>
Merge pull request #117 from abrt/gh116-rpm-build

make the build in the the build directory to not pollute the source tree

Copied from abrt commit 8efc696f2595805d877e0b48cb47b3d92caf99fa.

Closes #116.

Signed-off-by: Martin Milata <mmilata@redhat.com>