Merge bitcoin#20478: Don't set BDB flags when configuring without

982e548 Don't set BDB flags when configuring without (Jonas Schnelli)

Pull request description:

  Configuring `--without-bdb` on MacOS leads to a compile error (when BerkeleyDB is not installed). `brew --prefix berkeley-db4` always reports the target directory (even if not installed).

  This PR prevents BDB_CFLAGS (et al) from being populated when configuring `--without-bdb`

  ```
  ld: warning: directory not found for option '-L/Users/user/Documents/homebrew/Cellar/berkeley-db@4/4.8.30/lib'
  ld: warning: directory not found for option '-L/Users/user/Documents/homebrew/Cellar/berkeley-db@4/4.8.30/lib'
  ld: library not found for -ldb_cxx-4.8
  ld: library not found for -ldb_cxx-4.8
  ```

ACKs for top commit:
  promag:
    Tested ACK 982e548.
  hebasto:
    ACK 982e548, tested on macOS 11 Big Sur.

Tree-SHA512: f8ca0adca0e18e2de4c0f99d5332cba70d957a9d31a357483b43dcf61c2ed4749d223eabadd45fdbf3ef0781c6b37217770e9aa935b5207eaf7f87c5bdfe9e95

configure.ac

@@ -643,7 +643,7 @@ case $host in
 
          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$BDB_LIBS" = "x"; then
+         if test x$bdb_prefix != x && test "x$BDB_CFLAGS" = "x" && test "x$BDB_LIBS" = "x" && test "$use_bdb" != "no"; then
            dnl This must precede the call to BITCOIN_FIND_BDB48 below.
            BDB_CFLAGS="-I$bdb_prefix/include"
            BDB_LIBS="-L$bdb_prefix/lib -ldb_cxx-4.8"