CXXFLAGS ignored by g++

Building M4-1.4.19 consistently fails: gl_xlist.h issues half a dozen warnings about converting integers to pointers. In gl_list.h these are explicitly declared to be pointers to structs. I don't know why g++ treats them as integers and then treats warnings as an error.

I've set CXXFLAGS to "-std=gnu++2b -Wno-error=int-conversion" and sometimes "-w" but g++ seems to ignore these flags.