cmake: populate CMAKE_FIND_ROOT_PATH when cross-compiling for a different architecture on Linux

[illwieckz]

Populate CMAKE_FIND_ROOT_PATH when cross-compiling on Linux.

Fix:

• We may need to guess CMAKE_FIND_ROOT when cross-compiling #1659

[illwieckz]

I modified a bit the comments to make them more understandable.

[slipher]

Why is it inside if (DEPS_DIR) ?

[illwieckz]

Right, it's not needed to be there, I moved it out of that block.

[slipher]

It would be better to avoid setting CMAKE_FIND_ROOT_PATH if possible, since it causes CMAKE_PREFIX_PATH not to work as I discovered on #1433. When both of those variables are set, CMake concatenates the root path and prefix path together, instead of using the prefix path as is (unless the prefix path is already a subdirectory of the root path).