configure: do not make Capstone dependency automagic

configure: do not make Capstone dependency automagic

This adds regular ./configure options to control dependency on the
Capstone disassembly engine. See [0] for the rationale.

[0] https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Automagic_dependencies

Change-Id: I3e16dc5255d650aa1949ccf896b26dc96e522a75
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/5985
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>