Sorry, I missed your previous post before asking ...
Thanks for sharing your knowledge !

If I understand well regarding your example, *cmake* has to be *only* in 
*hostmakedepends* (and not makedepends) for cross-compiling on x86_64 for 
armv7 arch.

If not, I mean if cmake was listed in makedepends: 
call of cmake won't work, because its runtime will be an armv7 binary then, 
that can't be runned on x86_64 arch (which is cross-compiling).

So the rule for cross-building is: 
all stuff that need to be runtimed/executed during the build process (and 
not only linked/generated) has to be in *hostmakedepends* in order to have 
the right binary platform type (native and not cross-target architecture).

right ?