RFR: 8339128: Cannot resolve user specified tool properly after JDK-8338304

[Jiangli Zhou]

Please review this PR that fixes JDK build configure failure when using clang after JDK-8338304:


Found candidate GCC installation: <snip>
Selected GCC installation: <snip>
...
clang: error: unsupported option '-V -static-libgcc'
configure:85215: $? = 1
configure:85204: <snip> -qversion >&5
clang: error: unknown argument '-qversion'; did you mean '--version'?
...


Tool from Clang GCC installation is being picked up unexpectedly (JDK build configured with `--with-toolchain-type=clang`). With this fix, it avoids `UTIL_REQUIRE_PROGS(LLD, lld)` if user supplies compiler toolchain. The user supplied compiler toolchain is used by setting `CXX=<user_specified_path_to_tools_script>` during configuration.

-------------

Commit messages:
 - Don't do UTIL_REQUIRE_PROGS(LLD, lld) if user supplies compiler name.

Changes: https://git.openjdk.org/jdk/pull/21817/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21817&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339128
  Stats: 5 lines in 2 files changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21817.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21817/head:pull/21817

PR: https://git.openjdk.org/jdk/pull/21817