Print dummy column for MAPQ for mpileup -s

Thanks to Pasi Rastas for reporting this bug.
Disconnect clean-htslib from clean target

Make "clean" is now just a local clean; use "clean-all" to clean htslib
as well, as desired at meetings past.

Add default empty LDLIBS definition, previously forgotten.
Merge pull request #179 from mp15/stats

Stats unistd fix and tests enable