bug#78682: Building libltdl with MSVC fails

[Kirill Makurin]

Hi,

Building libltdl with MSVC fails:

```

libltdl_la-ltdl.obj : error LNK2019: unresolved external symbol lt_ltdl_LTX_preloaded_symbols referenced in function lt_dlinit

libltdl\.libs\libltdl-7.dll : fatal error LNK1120: 1 unresolved externals

```

I attached full invocation in separate file since it's quite messy.

I did some investigation.

Apparently, `lt_ltdl_LTX_preloaded_symbols` is supposed to be defined in libtool-generated `libltdl/.libs/libltdlS.c`, however it defines `lt_libltdl_LTX_preloaded_symbols` instead, as if it was mingw build.

I was building from libtool-2.5.4 release tarball. I tried building from `master` and the issue also exist there.

Thank you for working on MSVC-related issues. I have some other things to report/suggest.

- Kirill Makurin

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