fix(cmake): avoid really slow compile on emscripten

[henryiii]

This is really slow on emscripten cross-compiles to WebAssembly. I think it's safe just to hard-code it when cross-compiling to emscripten. (Thoughts welcome).

Description

Suggested changelog entry:

* Hardcode lto/thin lto for Emscripten cross-compiles.

[henryiii]

We might want to solve this error when LTO flags are missing:

[ 86%] Building CXX object tests/CMakeFiles/pybind11_tests.dir/test_stl_binders.cpp.o
cd /home/runner/work/pybind11/pybind11/tests && /usr/bin/c++ -DBOOST_ALL_NO_LIB -DPYBIND11_SIMPLE_GIL_MANAGEMENT -DPYBIND11_TEST_BOOST -DPYBIND11_TEST_EIGEN -Dpybind11_tests_EXPORTS -I/home/runner/work/pybind11/pybind11/include -isystem /opt/hostedtoolcache/Python/3.9.16/x64/include/python3.9 -isystem /home/runner/work/pybind11/pybind11/_deps/eigen-src -Os -DNDEBUG -std=c++11 -fPIC -fvisibility=hidden -Wall -Wextra -Wconversion -Wcast-qual -Wdeprecated -Wundef -Wnon-virtual-dtor -Werror -MD -MT tests/CMakeFiles/pybind11_tests.dir/test_stl_binders.cpp.o -MF CMakeFiles/pybind11_tests.dir/test_stl_binders.cpp.o.d -o CMakeFiles/pybind11_tests.dir/test_stl_binders.cpp.o -c /home/runner/work/pybind11/pybind11/tests/test_stl_binders.cpp
In file included from /home/runner/work/pybind11/pybind11/include/pybind11/stl.h:12,
                 from /home/runner/work/pybind11/pybind11/tests/test_stl.cpp:10:
/home/runner/work/pybind11/pybind11/include/pybind11/pybind11.h: In static member function ‘static pybind11::handle pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...) [with Func = test_submodule_stl(pybind11::module_&)::<lambda(const boost_opt_no_assign&)>; Return = int; Args = {const boost::optional<test_submodule_stl(pybind11::module_&)::NoAssign>&}; Extra = {pybind11::name, pybind11::scope, pybind11::sibling, pybind11::arg_v}]::<lambda(pybind11::detail::function_call&)>::_FUN(pybind11::detail::function_call&)’:
/home/runner/work/pybind11/pybind11/include/pybind11/pybind11.h:248:33: error: ‘*((void*)& args_converter +4)’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  248 |                 = cast_out::cast(std::move(args_converter).template call<Return, Guard>(cap->f),
      |                   ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  249 |                                  policy,
      |                                  ~~~~~~~
  250 |                                  call.parent);
      |                                  ~~~~~~~~~~~~
/home/runner/work/pybind11/pybind11/include/pybind11/pybind11.h:224:21: note: ‘*((void*)& args_converter +4)’ was declared here
  224 |             cast_in args_converter;
      |                     ^~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors
make[2]: *** [tests/CMakeFiles/pybind11_tests.dir/build.make:580: tests/CMakeFiles/pybind11_tests.dir/test_stl.cpp.o] Error 1

But first I'm fixing the missing LTO flag. :)

[henryiii]

@hoodmane, does this look okay? Emscripten should always use clang, which has lto & thin lto?

[hoodmane]

Emscripten should always use clang, which has lto & thin lto?

Yes, and it's often a problem that configuration stuff is super slow on Emscripten so it's great to hard code things that will be true on a very recent clang.