RFR: 8348286: [AIX] clang 17 introduces new warning Wtentative-Definitions which produces Build errors

[Joachim Kern]

We (SAP) try to introduce the ‘IBM Open XL C/C++ for AIX 17.1.2’ (based on clang 17) as a feasible compiler for jdk25, because in combination with the 17.1.3 runtime this would enable the support for ubsan.
Unfortunately, the new compiler comes along with a new set of compiler warnings turned into errors by -Werror.
One of the warnings is -Wtentative-definitions. It is fired when a variable definition in a header is found:
/jdk/src/java.desktop/share/native/libawt/awt/image/imageInitIDs.h:36:20: error: possible missing 'extern' on global variable definition in header [-Werror,-Wtentative-definitions]
   36 | IMGEXTERN jfieldID g_BImgRasterID;
      | ^
>From now on headers allow only extern declarations of variables. The definition must take place in a c/cpp file. This means e.g.
imageInitIDs.h:36:20
   36 extern jfieldID g_BImgRasterID;
and the corresponding c-File
   jfieldID g_BImgRasterID;

The other possible solution would be to compile everything with
-Wno-tentative-definitions
which could be set in flags-cflags.m4, if compiling with clang 17.

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Commit messages:
 - JDK-8348286

Changes: https://git.openjdk.org/jdk/pull/23236/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23236&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348286
  Stats: 6 lines in 1 file changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/23236.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23236/head:pull/23236

PR: https://git.openjdk.org/jdk/pull/23236