[CMake] Add CXX dependency generation to custom command for Ninja?

[Chris Green]

Hi,

We have a custom command for a third-party utility (genreflex, from the 
ROOT system) which has a directive:

     IMPLICIT_DEPENDS CXX ${CMAKE_CURRENT_SOURCE_DIR}/classes.h

How would I, either as an extra COMMAND or as a separate custom command, 
manually invoke the configured CXX compiler with all the 
correctly-configured include directories and other pre-processor and 
compiler flags with the expected option (-MMD for GCC, I think) on the 
above-mentioned classes.h file so that the resulting file can be used in 
a DEPFILE directive for the Ninja generator? I would like to model as 
closely as possible exactly what CMake would specify to invoke the 
compiler to (e.g.) compile an object with only the necessary changes, 
but I'm not sure whence to obtain this information. I'm assuming it's 
some combination of automatic variables and properties via generator 
expressions, but I'm at a loss, I'm afraid. I also don't know whether I 
would need to specify the .d file as an OUTPUT or a BYPRODUCTS if I do 
this as a COMMAND, or in DEPENDS in addition to DEPFILE if a separate 
custom command, and which choice would be better (assuming both of them 
work). The resulting CMake code would need to conditionally support both 
Makefile and Ninja generators.

Any advice gratefully accepted.

Current custom command definition:

   add_custom_command(
     OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${dictname}_dict.cpp
     ${SOURCE_OUTPUT} ${GD_ROOTMAP_OUTPUT} ${PCM_OUTPUT}
     COMMAND ${ROOT_GENREFLEX} ${CMAKE_CURRENT_SOURCE_DIR}/classes.h
     -s ${CMAKE_CURRENT_SOURCE_DIR}/classes_def.xml
		-I${CMAKE_SOURCE_DIR}
		${GENREFLEX_INCLUDES} ${GENREFLEX_FLAGS}
     -o ${dictname}_dict.cpp
     ${CLEANUP_COMMAND}
     IMPLICIT_DEPENDS CXX ${CMAKE_CURRENT_SOURCE_DIR}/classes.h
     DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/classes_def.xml
     COMMENT "Generating dictionary files for target ${dictname}"
     )

Many thanks,

Chris.


-- 
Chris Green <greenc at fnal.gov>, FNAL CS/SCD/ADSS/SSI/TAC;
'phone (630) 840-2167; Skype: chris.h.green;
IM: greenc at jabber.fnal.gov, chissgreen (AIM),
chris.h.green (Google Talk).