8374899: [8u] Fully handle clang as the toolchain in flags.m4

[gnu-andrew]

There are a number of cases in flags.m4 where the toolchain is checked and clang (now the default on all supported versions of XCode on MacOS) is not handled. In particular, this means -std=gnu++98 is never added and attempts to build with MacOS 15 fail as it is trying to meet the demands of C++11.

This PR updates the file to handle clang with gcc in most places as there is more commonality between the two than difference. It doesn't attempt to remove the gcc on MacOS support and so some areas will differ from 11u and later where MacOS is assumed to be clang-only.

The changes are as follows:

1. Handle linker flags under one shared gcc & clang block as there is mostly duplication between the two with the only difference being PICFLAG and PIEFLAG.
2. Handle the -O flag for both gcc & clang together as they only differ by OS.
3. Handle clang in FLAGS_SETUP_COMPILER_FLAGS_FOR_JDK
4. Move the Linux defines (-D_GNU_SOURCE -D_REENTRANT -D_LARGEFILE64_SOURCE) to where the BSD, MacOS & AIX ones are added. This presumably means these were not previously being added for Linux on clang. This mirrors 11u handling.

The changes in #3 mean that clang builds will now run the compiler tests for -std=gnu++98, -Wformat-overflow and -ffp-contract as well as the warning & -fstack-protector flags added in this block. Some of this may need to be exempted depending on testing.

As MacOS GHA builds do not currently run, I will make #738 depend on this PR to test there.

This also adds LOG_LEVEL=debug to the remaining OpenJDK GHA builds where it was absent (x86 Linux, Windows & Mac). This will mean noisier logs, but more useful ones when there is actually a failure. The current failing Mac ones don't tell us the commands being passed to the compiler. The same option has been present for x86_64 and alt Linux builds for some time.

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Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue
• JDK-8374899 needs maintainer approval

Issue

• JDK-8374899: [8u] Fully handle clang as the toolchain in flags.m4 (Bug - P4)

Reviewers

• Severin Gehwolf (@jerboaa - Reviewer)
• Sergey Bylokhov (@mrserb - Reviewer)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk8u-dev.git pull/740/head:pull/740
$ git checkout pull/740

Update a local copy of the PR:
$ git checkout pull/740
$ git pull https://git.openjdk.org/jdk8u-dev.git pull/740/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 740

View PR using the GUI difftool:
$ git pr show -t 740

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk8u-dev/pull/740.diff

Using Webrev

Link to Webrev Comment

[bridgekeeper]

👋 Welcome back andrew! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[gnu-andrew]

I think this may have to be a regular PR to generate the pr/740 branch.

[mlbridge]

Webrevs

• 02: Full (f82b5ef1)
• 01: Full - Incremental (a363af1d)
• 00: Full (a89902a9)

[openjdk-notifier]

The parent pull request that this pull request depends on has now been integrated and the target branch of this pull request has been updated. This means that changes from the dependent pull request can start to show up as belonging to this pull request, which may be confusing for reviewers. To remedy this situation, simply merge the latest changes from the new target branch into this pull request by running commands similar to these in the local repository for your personal fork:

git checkout JDK-8374899
git fetch https://git.openjdk.org/jdk8u-dev.git master
git merge FETCH_HEAD
# if there are conflicts, follow the instructions given by git merge
git commit -m "Merge master"
git push

[jerboaa]

This seems OK. Thanks!

[openjdk]

⚠️ @gnu-andrew This change is now ready for you to apply for maintainer approval. This can be done directly in each associated issue or by using the /approval command.