RFR: 8274980: Improve adhoc build version strings [v4]

[TheShermanTanker]

On Thu, 3 Feb 2022 12:56:12 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

>> Current adhoc version build strings are not ideal. Some of the problems:
>>  * A build number of "0" is inserted, which make the version string look like it's an official build, at least when not reading carefully
>>  * The version string gives little indication on what source code the build was based
>> 
>> Also, an error was discovered in how the build system generates version strings without a build number (since this basically never happened before). A version without a build number,  and without a PRE value, but with an OPT value, should have a "+-" separator, according to JEP 223. While this was correctly handled, the "+-" separator was also applied to versions without a build, but with both PRE and OPT. In this case, the "+" should be omitted, according to JEP 223. This bug is fixed by this commit.
>> 
>> Ideally, the adhoc version string should include something along the lines of the output of `git describe`. However, since the version string is set at configure time, not build time, this will almost immediately become misleading. :-( A substitute is proposed in this patch, where the branch name is included in the OPT string (if it's not `master`).
>> 
>> Finally, this patch fixes hotspot so it can properly build without a build number. This was the last blocker for why the build system always required the "0" as build number before. To facilitate interoperability with external tools (like jib) that still sets build number to "0" to really signal "no build number", a build number exactly matching "0" will be interpreted as having no build number.
>
> Magnus Ihse Bursie has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:
> 
>  - Merge branch 'master' into improve-adhoc-version-string
>  - IS_EMPTY define did not work on msvc
>  - Break long lines
>  - Skip setting git branch in adhoc version opt
>  - Fix for abtract_vm_version so it really handles empty VERSION_BUILD
>  - 8274980: Improve adhoc build version strings

I'm not sure if there's a neater way to do this, but I did come up with a fix for [IS_EMPTY define did not work on msvc](https://github.com/openjdk/jdk/pull/6152/commits/3c93c740e6d0d8624be6f87b5495d61b408167bd): https://github.com/magicus/jdk/commit/085ae29f2f01df1f06433713f5a554e7f3b375aa which seems to work properly for MSVC when building on Windows when handling empty version build numbers. I'm not too well versed with the build system though, so I'm not entirely sure that it working with MSVC is a direct result of the change or just a coincidence (Potentially caused by the change made to make/autoconf/spec.gmk.in in this PR)

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

PR: https://git.openjdk.java.net/jdk/pull/6152