ncursesw: specifically test if ncursesw/ncurses.h exists

in a sane ncurses environment, ncurses is built with widechar
support, but only features a single ncurses header.
the functionality is available if libncursesw* is there.
curses.h: need to define a magic flag to pull widechar support
add flag to disable debug mode
pick up CXXFLAGS passed on commandline
README: document the possibility to set custom CXXFLAGS etc