Surround the libraries with --start-group and --end-group to work
around <https://github.com/thepowersgang/mrustc/issues/138>.

diff --git a/src/trans/codegen_c.cpp b/src/trans/codegen_c.cpp
--- a/src/trans/codegen_c.cpp
+++ b/src/trans/codegen_c.cpp
@@ -960,6 +960,7 @@ namespace {
                     {
                         args.push_back("-L"); args.push_back(path);
                     }
+                    args.push_back("-Wl,--start-group");
                     for(const auto& lib : m_crate.m_ext_libs) {
                         ASSERT_BUG(Span(), lib.name != "", "");
                         args.push_back("-l"); args.push_back(lib.name.c_str());
@@ -975,6 +976,7 @@ namespace {
                     {
                         args.push_back("-l"); args.push_back(path.c_str());
                     }
+                    args.push_back("-Wl,--end-group");
                     for( const auto& a : Target_GetCurSpec().m_backend_c.m_linker_opts )
                     {
                         args.push_back( a.c_str() );