Merge #20563: build: Check that Homebrew's berkeley-db4 package is actually installed

d3ef947 build: Check that Homebrew's berkeley-db4 package is actually installed (Hennadii Stepanov)

Pull request description:

  On master (a0489f3) the `configure` script is not able to determine that Homebrew's `berkeley-db4` package is uninstalled. This causes a compile error on macOS.

  With this PR, and with the [uninstalled](https://stackoverflow.com/questions/20802320/detect-if-homebrew-package-is-installed) `berkeley-db4` package:
  ```
  % ./configure -q
  configure: error: libdb_cxx headers missing, Bitcoin Core requires this library for BDB wallet support (--without-bdb to disable BDB wallet support)
  ```

  Related #20478.

ACKs for top commit:
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    Tested ACK d3ef947.
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Tree-SHA512: 8dc532e08249ec63bd357594aa458d314b6e8537fc63f5b1d509c84d0d71d5b1f70172caa1a7efe2fc8af31c829e7982a0695cf3fbe5cbc477019550269915e1

Author: jonasschnelli

configure.ac

@@ -641,9 +641,9 @@ case $host in
          dnl It's safe to add these paths even if the functionality is disabled by
          dnl the user (--without-wallet or --without-gui for example).
 
-         bdb_prefix=$($BREW --prefix berkeley-db4 2>/dev/null)
          qt5_prefix=$($BREW --prefix qt5 2>/dev/null)
-         if test x$bdb_prefix != x && test "x$BDB_CFLAGS" = "x" && test "x$BDB_LIBS" = "x" && test "$use_bdb" != "no"; then
+         if $BREW list --versions berkeley-db4 >/dev/null && test "x$BDB_CFLAGS" = "x" && test "x$BDB_LIBS" = "x" && test "$use_bdb" != "no"; then
+           bdb_prefix=$($BREW --prefix berkeley-db4 2>/dev/null)
            dnl This must precede the call to BITCOIN_FIND_BDB48 below.
            BDB_CFLAGS="-I$bdb_prefix/include"
            BDB_LIBS="-L$bdb_prefix/lib -ldb_cxx-4.8"