[CMake] Do not build targets whose dependencies are not available

[Matthew Woehlke]

On 2013-03-28 16:02, Chris Stankevitz wrote:
> On Tue, Mar 19, 2013 at 3:13 AM, Ansis Māliņš wrote:
>>>
>>> Q: Can cmake automatically not build "b" when "a" is not available?
>>
>> find_package(a)
>> if(a_FOUND)
>>      ExternalProject_Add(b PREFIX blabla ...)
>> endif()
>
> Hello,
>
> If I understand correctly, this is what just happened:
>
> Chris: Is there a pretty solution to problem X?  An ugly solution is Y.
>
> Ansis: Why don't you try Y?
> [snip example]

First, I would suggest putting your dependent components in their own 
subdirectories. Then you can at least do:

if(Boost_FOUND)
add_subdirectory(b)
endif()


If even that bothers you, you could wrap the above in a macro like:

macro(optional_add_subdirectory dir)
   if(${ARGN})
     add_subdirectory(${dir})
   endif()
endmacro()

Note the use of ARGN to allow for things like:

optional_add_subdirectory(b Boost_FOUND AND ENABLE_B)
optional_add_subdirectory(c NOT DISABLE_C)


Second:

if(a_FOUND AND b_FOUND AND ...)


I'm not aware of any built-in way of doing this sort of thing 
automatically. The best you can do I think is going to be to use 
functions and/or macros to make things more convenient.

It's an interesting notion, though...

-- 
Matthew