Enforcing JDK 1.4-specific modules

[Marc Prud'hommeaux]

We recently had some problems where JDK 1.5-specific methods (e.g.,  
String.contains(String)) were slipping into JDK 1.4-specific modules.  
This isn't a problem for stand-alone OpenJPA (whose only  
implementation is JPA, which is JDK 1.5-dependent), but other  
products built on OpenJPA may still support JDK 1.4.

To that end, I've added a 'java14.jar' property to the pom.xml files  
that can be used to specify the location of the JDK 1.4 runtime jar,  
against which we will compile the JDK 1.4-specific modules. Since the  
location of the JDK 1.4 runtime jar is platform- and installation-  
dependent, we can't just rely on a single location (nor is there any  
maven-friendly JDK 1.4 runtime jar that I am aware of), the flag  
needs to be specified manually.

Note that the flag will be silently ignored when it is unspecified,  
which I expect will be the common case for most OpenJPA developers.  
You only need to use it if you are interested in ensuring that any  
code you add to JDK 1.4 modules won't break 1.4 compatibility.