gdbmmodule and dbmmodule need special treatment anyway. macOS has dbm-API build into libc. Linux has either libgdbm_compat, libndbm, or libdb. The --with-dbmliborder makes it even more interesting. Users can override in which order they want to probe for gdbm, ndbm, and libdb. We have to keep the "manual" library and header checks.

Cross-compilation with pkg-config uses a trivial wrapper script, https://autotools.io/pkgconfig/cross-compiling.html . The build system has to create and provide a script with correct sysroot setting.

Is traditional cross-compiling work still useful these days anyway? Emulation has made big leaps in the last decade. Emulated cross compiling with qemu has become widespread. It also has the big advantage that you can run the test suite on the emulated hardware and verify that your binaries work. AFAIK Fedora's build system uses qemu for a bunch of hardware targets.