build: Add `-Wundef`

[hebasto]

This PR addresses https://github.com/bitcoin/bitcoin/pull/29876#issuecomment-2157739910:

Steps to reproduce an issue:

$ g++ -Wundef -Wno-unused-parameter -o src/fields/libminisketch_a-generic_4bytes.o -c src/fields/generic_4bytes.cpp
In file included from src/fields/generic_common_impl.h:12,
                 from src/fields/generic_4bytes.cpp:12:
src/fields/../int_utils.h:162:7: warning: "HAVE_CLZ" is not defined, evaluates to 0 [-Wundef]
  162 | #elif HAVE_CLZ
      |       ^~~~~~~~

[sipa]

utACK 978a3d886995a84ef5b84693e815c6382c9ec820