Install data to CDOGS_DATA_DIR

[reinerh]

utils.c is searching data files in CDOGS_DATA_DIR, so make sure that the files are also installed there (similar to CDOGS_DOC_DIR).

[cxong]

DATA_INSTALL_DIR is also referred in other places - see https://github.com/search?q=repo%3Acxong%2Fcdogs-sdl%20DATA_INSTALL_DIR&type=code
I think we should just replace it with CDOGS_DATA_DIR - and move the relevant code up here

cdogs-sdl/CMakeLists.txt

Lines 138 to 145 in b8e61bb

	if(NOT DEFINED CDOGS_DATA_DIR)
	if(GCW0)
	set(CDOGS_DATA_DIR "./")
	else()
	set(CDOGS_DATA_DIR "../")
	endif()
	endif()
	message("Data dir is ${CDOGS_DATA_DIR}")

[cxong]

It looks like this value can be different to CDOGS_DATA_DIR due to various packaging scenarios, but I don't remember how this works. In any case we would need to test the packaging in multiple platforms to make sure this works, so would it be easier to just set DATA_INSTALL_DIR to the right value?

[cxong]

7fa536b
this change may be relevant, for setting DATA_INSTALL_DIR