Makefile: use "pkg-config --exists"

Don't fail if a library provides neither CFLAGS nor LIBS.
Dialogs/WaypointDetails: remove obsolete code

This code has been disabled for several years and has become
completely obsolete meanwhile.
Android/TextUtil: remove explicit NULL initialisation

Global variables are initialised to NULL implicitly, and this was
redundant.
Android: use nullptr instead of NULL
ActionInterface: use nullptr instead of NULL
Blackboard/RateLimitedBlackboardListener: use nullptr instead of NULL
Device: use nullptr instead of NULL