* `AX_LIB_EXPAT` has a big practical issue: It tries to detect expat
  by injecting various paths, among which is /usr/lib. This is extremely
  problematic, as /usr/lib usually contains 32-bit libraries on 64-bit
  systems and thereby kills linkers such as LLD that reject 32-bit
  binaries when linking 64-bit binaries. `pkg-config` does not inject
  /usr/lib, and can also be overridden at configure time.

Closes: https://bugs.gentoo.org/737974