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.