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

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"