Make it possible to target any version of ICU

Modified Makefile to include a ICU_VER argument
Modified README to specify XX instead of 49 as the target DLL version

ICU_VER is now a macro definition either made in dummy_icu.h or
from the command line (Makefile)

Added a set of macros for declaration with suffixing and concatenation
of the ICU version for the NOP functions.

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