net/tigervnc: Switch to options helpers

• Cleanup unnecessary CFLAGS
• Switch shell substitution from `` to $()
• Switch to ImageMagick 7 (used only to create icons during build)
• s/XORG_WRKDIR/XORG_WRKSRC/ as the variable actually refers WRKSRC
• Use relative path to refer other ports as many other ports do that

Reviewed by: adamw
Differential Revision: https://reviews.freebsd.org/D19585