PROJ 6.0.0 has been released and it deprecates the proj_api.h interface that spatialite_gui uses.

Including that header without using the -DACCEPT_USE_OF_DEPRECATED_PROJ_API_H=1 flag causes an error:

 error: #error 'To use the proj_api.h you must define the macro ACCEPT_USE_OF_DEPRECATED_PROJ_API_H'
  #error 'To use the proj_api.h you must define the macro ACCEPT_USE_OF_DEPRECATED_PROJ_API_H'
   ^~~~~

PROJ 6.0.0 removes the old projects.h interface, and PROJ 7.0.0 (scheduled for March 202) will remove proj_api.h.

Support for the proj.h API introduced in PROJ 6.0.0 should be added to spatialite-gui.