build: no-longer fail default configure if BDB isn't available

Inline with moving to descriptor (sqlite) wallets by default for 0.23,
this adapts the build system so that a default `./configure` invocation
no-longer fails if BDB isn't present. Currently, if configure is run
with no options, and no BDB is present, we'll fail with:
```bash
checking for Berkeley DB C++ headers... no
configure: error: libdb_cxx headers missing, Bitcoin Core requires this library for BDB wallet support (--without-bdb to disable BDB wallet support)
```

If descriptor wallets are to be the default, this behaviour no longer
makes sense, as a builder should be able to configure and build, to use
a wallet, without BDB installed, and without passing additional
arguments, i.e `--without-bdb` or `--with-incompatible-bdb`, to
configure.

With this change, running configure will no-longer fail, and will
instead print:
```bash
checking for Berkeley DB C++ headers... no
configure: WARNING: libdb_cxx headers missing
configure: WARNING: Bitcoin Core requires this library for BDB (legacy) wallet support
configure: WARNING: Passing --without-bdb will suppress this warning
checking for sqlite3 >= 3.7.17... yes
checking whether to build wallet with support for sqlite... yes
```

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