m4/libtool.m4: fix emscripten CXX postdeps using non-PIC sysroot

m4/libtool.m4: fix emscripten CXX postdeps using non-PIC sysroot

Commit d8a934458b251b465b7d31da9ba9148a128a146d introduced shared
libraries support for emscripten, using -sSIDE_MODULE=1, but the CXX
dependency detection in _LT_SYS_HIDDEN_LIBDEPS will only try to use
`-shared` link option to discover implicit dependencies.

The libraries and search paths reported by em++ differ depending on
whether -sSIDE_MODULE=2 is passed:

  em++ -shared -v conftest.o:
    -L<sysroot>/lib/wasm32-emscripten  (<-- non-PIC)
       -lhtml5... (other -l flags)

  em++ -sSIDE_MODULE=2 -shared -v conftest.o
    -L<sysroot>/lib/wasm32-emscripten/pic  (<-- PIC)
       (no -l flags)

Without -sSIDE_MODULE=2, the detected postdeps contain the non-PIC
sysroot path and system libraries. These get injected into the CXX
shared library link line before emcc can append the PIC sysroot,
causing wasm-ld to resolve system libraries from non-PIC archives,
typically in VLC build system:

    error: R_WASM_MEMORY_ADDR_LEB cannot be used against symbol;
           recompile with -fPIC

We resolved the issue in VLC by patching the postdeps but they were not
generated properly in the first place.

Fix by overriding output_verbose_link_cmd for emscripten in
_LT_LANG_CXX_CONFIG. The override passes -sSIDE_MODULE=2, matching what
archive_cmds actually uses at link time so that the detected paths point
to the PIC sysroot.

* NEWS: Update.