To avoid the drawback, we can put the logic of locating possible libdir in gpg-error.m4, instead of putting in the script.

IIUC, in Gentoo multilib (or other distributions), <triplent>-{gpg-error,libgcrypt,libassuan,npth,libksba,npth}-config script is used.
In forthcoming libgpg-error 1.33, single gpgrt-config is used for all architecture, by having --libdir option at invocation time.

New gpg-error.m4 detects gpgrt-config, too.
And configure supplies --libdir when it invokes gpgrt-config.
For other *.m4 (libassuan, ksba, libgcrypt, ntbtls), it is possible for them to check GPGRT_CONFIG to use gpgrt-config if any.
For npth.m4, it can do that too, with no hard dependency to libgpg-error.

I decided to change gpgrt-config to have --libdir option.
By supplying libdir directly, it's no need anymore to detect the directory by CC variable.
gpg-error.m4 is also updated.

I need more information:

• where is pkg-config path for <host_alias>? How is it determined?
  • 32-bit: /lib or /lib32?
  • 64-bit: /lib or /lib64?
  • something like x32: where???

I consider:

• Single gpgrt-config is better (and simpler)
• new option --for-host=<host_alias>? (--host is already used for query for host)
• update *.m4 using this new option to provide host information to determine the path

libassuan: Done
libksba: Done
npth: Done
ntbtls: Done
libgcrypt: Done

A bit tricky, but this would be good to use gpgrt-config by gpg-error.m4.
I say "tricky", because its name is gpg-error.m4 but it configure GPGRT_CONFIG to access to GPG_ERROR_CONFIG.

It might be good idea to provide libgcrypt.pc in libgcrypt 1.8.x for forward compatibility with libgpg-error 1.33.

Well, I changed my mind. Use of new gpgrt-config requires software update to introduce gpgrt.m4 and update of configure.ac to switch gpgrt from gpg-error, in standard way.
That's too much this time. It's good to defer this change.

Sorry, there is no good way, but only workaround in this case, because it is complicated and it is basically wrong thing to do it by composite device (in my opinion). I'll describe detail in this comment.
I'm not Windows user, so, I don't have an idea how to recover from such a situation.

OK, I'll change to use gpgrt-config, along with requiring newer version of libgpg-error.