Use -D_CONST_X_STRING to clear many const warnings
When built with a libXt that makes the
String type be
const char * when
_CONST_X_STRING is defined, this removes 62 of the 98 -Wdiscarded-qualifiers warnings from gcc 7.
When built with an older libXt, things are no worse than before.
cc: @dickey (who introduced
_CONST_X_STRING to libXt)