Fix redefinition of `clock_gettime` and `clock_getres`

	winpthreads-git 12.0.0.r720 provides `clock_gettime` and
	`clock_getres` as inline functions.

	digest.so needs ruby/digest.h which is installed by build-ext

	Copy to path with the base name

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.

By replacing `ALLOBJS` suffix with intermediate file suffixes instead
of roughly removing by wildcards.  Made `cleanlibs` append `.dSYM`
suffix for each word in `TARGET_SO`, not the end of the entire list.

This reverts commit 0ae0a0c1c3bb01d118c5a924d731a2b3e9774105.

This reverts commit 71d511615b8dac459d6f88f5896b9859f99c999e.

The `POSTLINK` variable had been used in the `LINK_SO` variable, which
is used to link shared extension libraries. However, the `POSTLINK`
variable had not been defined in the generated Makefile, so extension
libraries were not properly post-processed. It was not a critical issue
for the existing `POSTLINK` usage for darwin platforms, but it would be
a problem for Wasm/WASI platform, which requires *mandatory*
post-processing for shared extension libraries.

"AR" was well propagated to the enc.mk and mkmf, but "LD" was not. This
caused the dynamic libraries to be linked with a linker found in the PATH,
which could be different from the one used in the Ruby build process.
This is especially important for cross-compilation, where the host
linker may not be compatible with the target system. (e.g. WebAssembly)