consider .S as a source file

it's asm that needs to be linked with the final archive/exe

this is gcc-centric / not MS compatible

src/dds/sdist/file.cpp

     assert(std::is_sorted(header_exts.begin(), header_exts.end()));
     static std::vector<std::string_view> source_exts = {
         ".C",
+        ".S",
         ".c",
         ".c++",
         ".cc",