update build requirements

in particular, mention perl-related things, which might not be
immediately obvious.

README

@@ -55,11 +55,18 @@ isync executable still exists; it is a compatibility wrapper around mbsync.
 
 * Requirements
 
+    perl v5.14+
     Berkeley DB 4.1+ (optional)
     OpenSSL for TLS/SSL support (optional)
     Cyrus SASL (optional)
     zlib (optional)
 
+  The build from git also requires:
+
+    GNU autotools (autoconf & automake)
+    perl module Date::Parse (libtimedate-perl on Debian, perl-TimeDate on
+                             Fedora and Suse)
+
 * Installation
 
     ./autogen.sh (only when building from git)