the minimum required bdb version is in fact 4.1

this is the one that introduced the transaction argument to db->open().

README

@@ -61,7 +61,7 @@ isync executable still exists; it is a compatibility wrapper around mbsync.
 
 * Requirements
 
-    Berkley DB 4.2+ (optional)
+    Berkley DB 4.1+ (optional)
     OpenSSL for TLS/SSL support (optional)
 
 * Installation

configure.ac

@@ -139,7 +139,7 @@ if test "x$ob_cv_with_sasl" != xno; then
 fi
 AC_SUBST(SASL_LIBS)
 
-AC_CACHE_CHECK([for Berkley DB >= 4.2], ac_cv_berkdb4,
+AC_CACHE_CHECK([for Berkley DB >= 4.1], ac_cv_berkdb4,
   [ac_cv_berkdb4=no
     sav_LDFLAGS=$LDFLAGS
     LDFLAGS="$LDFLAGS -ldb"