Merge branch 'next'

* next:
  minor, kick useless fini_hmap() and init_hmap()
  fix, PEBKAC, upon png error, destroy the write struct and bugger off without writing anything
  chore, guard png code against the case libpng isn't installed
  chore, check for libpng in c.ac
  minor, link command binaries marked with LD_EXPORT_DYNAMIC dynamically (kick -static)
  chore, don't link DSOs with libuterus, instead rely on ltdl's -export-dynamic feature
  chore, up the version number of libuterus, as it contains libm30 too now
  major, ditch libm30 again, instead put m30 string reader/writer into libuterus
  minot, use black but fully alpha for 0 returns
  fix, PEBKAC, also populate the main diagonal of the hmap
  minor, use libpng error handling by setting the png jumpbuf in prnt_hmap()
  minor, fill the hold map separately and parameterise its size
  minor, print hold maps in png format upon ute-anal
  minor, use libpng to write a return analysis to a .png file
  minor, provide command ute-anal for simple tseries analyses