Okay, so I'm a moron....

configure.in

@@ -1860,7 +1860,7 @@ AC_SUBST(MKSHLIB_UNFORCE_ALL)
 AC_SUBST(DSO_CFLAGS)
 AC_SUBST(DSO_LDOPTS)
 AC_SUBST(DLL_SUFFIX)
-AC_DEFINE(MOZ_DLL_SUFFIX, ".$DLL_SUFFIX")
+AC_DEFINE_UNQUOTED(MOZ_DLL_SUFFIX, ".$DLL_SUFFIX")
 
 if test "$USING_HCC"; then
    CC='${topsrcdir}/build/hcc'