Add configure warning message if ARCH environment variable is set

[maskimko]

Motivation and Context

I encountered an nasty issue when building zfs in the docker container, where environment variable ARCH was set to 'x86_64' value, and it was used for non zfs build related purpose.
Then I got the configure error message like this one:

checking for kernel config option compatibility... done
checking whether CONFIG_MODULES is defined... no
configure: error:
*** This kernel does not include the required loadable module
*** support!
***
*** To build OpenZFS as a loadable Linux kernel module
*** enable loadable module support by setting
*** CONFIG_MODULES=y in the kernel configuration and run
*** make modules_prepare in the Linux source tree.
***
*** If you don't intend to enable loadable kernel module
*** support, please compile OpenZFS as a Linux kernel built-in.
***
*** Prepare the Linux source tree by running make prepare,
*** use the OpenZFS --enable-linux-builtin configure option,
*** copy the OpenZFS sources into the Linux source tree using
*** ./copy-builtin <linux source directory>,
*** set CONFIG_ZFS=y in the kernel configuration and compile
*** kernel as usual.

It confused me, and I spent some time debugging the possible modules build issues.
The fact that on my VMs the same configuration of ZFS built well, initially led me to a wrong investigation path.
hence to save time of other users I'd like to introduce a warning message during the configure phase.

Description

This change adds autotools macros to the config/always-arch.m4 file, which produces the warning message if the ARCH variable is set in the building environment. This helps to avoid issues described above.

How Has This Been Tested?

When ARCH variable is set to some unsupported by kernel Makefile value, like ARCH="x86_64", I got this warning and failure:

checking for conflicting environment variables... warning
configure: WARNING: ARCH environment variable is set to "x86_64". This can cause build kernel modules support check failure. Please unset it.

checking for kernel config option compatibility... done
checking whether CONFIG_MODULES is defined... no
configure: error:
*** This kernel does not include the required loadable module
*** support!
***
*** To build OpenZFS as a loadable Linux kernel module
*** enable loadable module support by setting
*** CONFIG_MODULES=y in the kernel configuration and run
*** make modules_prepare in the Linux source tree.

If ARCH variable is unset I got:

checking for conflicting environment variables... done

checking for kernel config option compatibility... done
checking whether CONFIG_MODULES is defined... yes
checking whether CONFIG_BLOCK is defined... yes

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)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• 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.

[maskimko]

This is a duplicate of #17678, but with formatted commit to pass check style

[behlendorf]

Thanks for running this down. Just a few comments.