.cxx files can't be used as sources for shared libraries

[niclasr]

I was trying to compile a lib with a .cxx file as a source, like this:

LIB=mylib
SRCS=myclass.cxx
SHLIB_MAJOR=0
.include <mkc.lib.mk>

This breaks with a message like "don't know how to make mylib.os" .

[niclasr]

Fix merged, closing this.