Build: Discover.scala can use wrong includes in certain cases.

[LeeTibbert]

Build Discover.scala uses the wrong includes when an alternate clang and/or clang++
are specified.

The configuration below used to debug/verify in Issue #2779. Skip
lightly over the bug that the system linker is used lld instead of lld-15,
that is not the fault of Build.scala.

   nativeConfig ~= { _
        .withLTO(scala.scalanative.build.LTO.thin)
        .withLinkingOptions(_ :+ "-fuse-ld=lld")
        .withClang(Paths.get("/opt/homebrew/opt/llvm@15/bin/clang"))
        .withClangPP(Paths.get("/opt/homebrew/opt/llvm@15/bin/clang++"))
    }

When the above executes, it looks like Discover.scala Line 53 executes

     val llvmIncludeDir =
        Try(Process("llvm-config --includedir").lineStream_!.toSeq)
          .getOrElse(Seq.empty)

This gives the configuration for the installed system clang, say clang-17, and not
the requested clang. A similar problem exists at line 78.

This is an insidious bug because there may well be a mismatch between the
include and libraries files implied by the use of the alternate compiler and
those actually used. One may see either false positives or false negatives.