5.7.6beta18

Everything: Now fully compatible with PROJ 6.X. The configure script will detect
the presence or absence of PROJ 6 or later - if the PROJ installation predates
PROJ 6 then the old header files and API are used. PROJ 6 and later include a
comprehensive database of coordinate reference systems (e.g. projected coordinate
systems and/or datums), making the projected coordinate system list kept in the
file src/share/Projections.dat unnecessary. For now this file will continue to
be part of the MB-System distribution to support installation with obsolete
versions of PROJ.

mbtrnpp: Removed some unnecessary test data and code from the src/mbtrnav/gctp/
directory.

mbtrnpp: Fixed a number for formatting and type issues hampering building the
TRN code on MacOs.