CMake's exec_program is deprecated, revert back

This was changed in commit aa13f3 to use exec_program, but this
function is deprecated and execute_process should be preferred.

Change-Id: I45a85307c27eb204082f0176f28fff432af57257
Merge topic 'cmake-dep-funcs' into master

d350387f CMake's exec_program is deprecated, revert back