97a4300 Planner: use time.h and utc_mkti...

based on: http://github.com/torvalds/subsurface/commit/97a43003c461427b99e1aa0dfc165841bdeaf90e

compiled with: mingw 3.4.5

notes:
Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>