boost build: pass cxxflags, add verbose logging
new GroupSortingWriter processor

This processor will cache vcf entries until endGroup() is called. At
that point, it will sort the entries in the cache, pass them on to
the next processor, and clear the cache to prepare for the next group.
fix processors CMake file

it was missing some headers
vcf-annotate & vcf-merge now write sorted output

they were having problems with this before. Now, each group of
overlapping entries gets sorted just before being printed.