Speed-up dramatically proj.db build time.

[rouault]

(follow-up / improvement of #4279)

Current proj.db build time is typically 50 to 60 seconds (and up to 7.5 hours on arm64 cross-compilation with full emulation!). Most of it is due to running consistency checks. Those checks actually only need to run once each time when we update the content of the database. When skipping them, the build time is cut to 3 seconds or so.
So in data/CMakeLists.txt, let keep track of an expected md5sum resulting from the concatenation of the data/sql/*.sql files. When building proj.db, we check if the got and expected md5sum match. If they do build proj.db by inserting the consistency check triggers after having inserted data record. If there's a mismatch, do a one time build with the triggers inserted before the data records, check that proj.db builds fine with that, and if so, emit a CMake error message indicating to the user that they must update the PROJ_DB_SQL_EXPECTED_MD5 variable in data/CMakeLists.txt with the provided value. Next runs will go through the fast build path, until content is updated again.

Demo of first time with outdated value in PROJ_DB_SQL_EXPECTED_MD5:

CMake Warning at generate_proj_db.cmake:35 (message):
  all.sql.in content has changed.  Running extra validation checks when
  building proj.db...


CMake Error at generate_proj_db.cmake:43 (message):
  Update 'set(PROJ_DB_SQL_EXPECTED_MD5 ...)' line in data/CMakeLists.txt with
  9a6b21de7b18f68719acb2260c3492fb value