Use _CONST_X_STRING to make libXt declare String as const char *
Clears 4 out of 24 -Wdiscarded-qualifiers warnings from gcc 7.3 (The rest come from libX11's XrmOptionDescRec defined in Xresource.h.)
Clears 4 out of 24 -Wdiscarded-qualifiers warnings from gcc 7.3 (The rest come from libX11's XrmOptionDescRec defined in Xresource.h.)