windows: fix linking .rc to shared curl with autotools

windows: fix linking .rc to shared curl with autotools

`./configure --enable-shared --disable-static` fails when trying to link
a shared `curl.exe`, due to `libtool` magically changing the output
filename of `windres` to one that it doesn't find when linking:

```
/bin/sh ../libtool --tag=RC --mode=compile windres -I../../curl/include -DCURL_EMBED_MANIFEST  -i ../../curl/src/curl.rc -o curl.o
libtool: compile:  windres -I../../curl/include -DCURL_EMBED_MANIFEST -i ../../curl/src/curl.rc  -o .libs/curl.o
[...]
CCLD     curl.exe
clang: error: no such file or directory: 'curl.o'
```

Let's resolve this by skipping `libtool` and calling `windres` directly
when building `src` (aka `curl.exe`). Leave `lib` unchanged, as it does
need the `libtool` magic. This solution is compatible with building
a static `curl.exe`.

This build scenario is not CI-tested.

While here, delete an obsolete comment about a permanent `libtool`
warning that we've resolved earlier.

Regression from 6de7322c03d5b4d91576a7d9fc893e03cc9d1057

Reported-by: Christoph Reiter
Fixes #9803
Closes #9805