Add CMake build

[headupinclouds]

• add CMake build files w/ standard package-config install
  i.e., end user can include in separate project w/
  find_package(pico CONFIG REQUIRED)
  target_link_librari(some_app pico::pico)
• use Hunter (optional) for OpenCV automatic dependency in app
  • use standard opencv include paths
  • add missing *_c.h includes for latest OpenCV
• use stdlib.h for defined(MACH) -- no malloc.h
• add OpenMP platform check for building