build: Fix the LINKER variable. The LINKER variable is set in configure.ac as either 'CC' or 'CXX' where it is then passed to the created Makefile. This fails with slibtool which can't find the 'CC' file and can be fixed by correctly setting the LINKER variable to an actual Makefile variable like '$(CC)' or '$(CXX)' instead. Presumably GNU libtool does some magic here to hide the issue.