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

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

 /usr/include/proj_api.h:37:2: 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.

check_sql_stmt also fails with PROJ 6.0.0:

Test case: ST_TransformXY - null Geometry
Test case: ST_TransformXY - 2D XY GeometryCollection
Unexpected value at 1: GEOMETRYCOLLECTION(POINT(1676380.538791 4551993.304759), LINESTRING(1684498.32413 4563303.102441, 1692590.049453 4574622.920763), POLYGON((1692590.049453 4574622.920763, 1734455.803022 4575853.343594, 1732655.00687
 4631372.543297, 1691111.320325 4630139.503573, 1692590.049453 4574622.920763), (1700655.635838 4585952.766718, 1725736.586454 4586691.348854, 1724694.769273 4620002.530275, 1699729.762459 4619263.006007, 1700655.635838 4585952.766718)))
|
Expected value was   : GEOMETRYCOLLECTION(POINT(1676380.538791 4551993.304759), LINESTRING(1684498.324131 4563303.102442, 1692590.049453 4574622.920763), POLYGON((1692590.049453 4574622.920763, 1734455.803025 4575853.343594, 1732655.0068
72 4631372.543298, 1691111.320326 4630139.503574, 1692590.049453 4574622.920763), (1700655.635839 4585952.766718, 1725736.586456 4586691.348854, 1724694.769275 4620002.530275, 1699729.76246 4619263.006007, 1700655.635839 4585952.766718))
)|
FAIL check_sql_stmt (exit status: 255)

full PROJ.6 support has been implemented by commit b72ce4b2a5