make twm compile with a c++ compiler

My editor's syntax highlighting does not differentiate between c and c++ code. Variables named "free", "new", "delete", or "class" get highlighted as keywords. Using c++ keyword names as variable names can also be confusing to humans -- at least for me.

After choosing different names, e.g. "remove" instead of "delete", twm was very close to be compile-able with a c++ compiler, so I added explicit casts to malloc calls and removed register specifiers as well.

I wouldn't know why someone would like to compile twm with a c++ compiler instead of c, but CC=c++ ./autogen.sh && make now works.