linux: detect if kernel defines intptr_t

[robn]

Motivation and Context

Since Linux 6.7 the kernel has defined intptr_t. Clang 18 has -Wtypedef-redefinition by default, which causes the build to fail because we also have a typedef for intptr_t.

Description

Adds some autoconf rules to check kernel types, then checks for intptr_t. We define our own if we didn't find out.

Typical error before (clang-18):

make[3]: Entering directory '/home/robn/code/quiz/build/kernel/linux-6.7.12'
  CC [M]  /home/robn/code/zfs/module/os/linux/spl/spl-condvar.o
In file included from /home/robn/code/zfs/module/os/linux/spl/spl-condvar.c:26:
In file included from /home/robn/code/zfs/include/os/linux/spl/sys/condvar.h:29:
In file included from /home/robn/code/zfs/include/os/linux/spl/sys/mutex.h:27:
/home/robn/code/zfs/include/os/linux/spl/sys/types.h:41:16: error: redefinition of typedef 'intptr_t' is a C11 feature [-Werror,-Wtypedef-redefinition]
   41 | typedef long                    intptr_t;
      |                                 ^
./include/linux/types.h:43:16: note: previous definition is here
   43 | typedef long                    intptr_t;
      |                                 ^
1 error generated.
make[5]: *** [scripts/Makefile.build:243: /home/robn/code/zfs/module/os/linux/spl/spl-condvar.o] Error 1

How Has This Been Tested?

Compiled against 6.4.x and 6.7.x with GCC 13 and Clang 18. All seems well.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.