Add support for PROJ 8.0.0 in FindProj.cmake.

proj_api.h is no longer available in PROJ 8.0.0, use proj.h instead.

Fixes: #41735

Author: sebastic

cmake/FindProj.cmake

@@ -40,6 +40,12 @@ FIND_PATH(PROJ_INCLUDE_DIR proj_api.h
   "$ENV{INCLUDE}"
   "$ENV{LIB_DIR}/include"
   )
+IF (NOT PROJ_INCLUDE_DIR)
+  FIND_PATH(PROJ_INCLUDE_DIR proj.h
+    "$ENV{INCLUDE}"
+    "$ENV{LIB_DIR}/include"
+    )
+ENDIF (NOT PROJ_INCLUDE_DIR)
 
 FIND_LIBRARY(PROJ_LIBRARY NAMES proj_i proj PATHS
   "$ENV{LIB}"