Support separate build directories.

The reference to "$(top_srcdir)" was present since commit e186c0a,
but needs to be "$(top_builddir)" in order to support out-of-source
builds.

I'll accept the responsibility for not catching this during review.
Correct a number of lingering white-space issues

This commit adopts the common Automake formatting conventions of
other OPM modules.
Merge pull request #27 from bska/clean-build

Clean build