Newer versions of clang's LSAN uses LSAN_OPTIONS environment variable
instead of ASAN_OPTIONS.

Describe two features that were missing from the documentation:

- yields configuration part if a block is given.
- "depend" file will be included.

1. Store the `PKG_CONFIG` variable in Makefile.sub (or try to get it from the ENV var PKG_CONFIG in mkmf.rb)
2. Try to use --msvc-syntax, with a fallback to replacing -Lxxx with -libpath:xxx. --msvc-syntax has been in pkgconf since 1.4.0 (released 7 years ago). pkg-config (freedesktop), does not support it, hence the fallback.
3. The `try_ldflags` passes these `ldflags` as the `opt` parameter to the `link_command`, not as `ldflags`. Unix systems are forgiving in that regard, MSVC is not: as a result as passing them as `opt`, they (specifically the `/libpath:xxx` ones) end up passed before the `-link` command to `cl.exe` and it throws because it ignores it and therefore can't find the lib.

```
cl : Command line warning D9002 : ignoring unknown option '-libpath:C:/Users/julien/.conan2/p/libff3726d89a6255c/p/lib'
```

Join with a space in `MakeMakefile#libpathflag` instead.

Introduce a new mkmf option `--target-rbconfig` to specify the RbConfig
file for the deployment target platform. This option is useful for
cross-compiling Ruby extensions without faking the global top-level
`RbConfig` constant.