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.

configure.ac

@@ -84,10 +84,10 @@ esac
 dnl use CXX for linker on Haiku
 case "$host" in
     *-*-haiku*)
-        LINKER=CXX
+        LINKER='$(CXX)'
         ;;
     *)
-        LINKER=CC
+        LINKER='$(CC)'
         ;;
 esac
 AC_SUBST(LINKER)