how to exclude directories from maven build cache extension inputs

[Vincent Boulaye]

Hello,


I am trying to use maven-build-cache-extension to speed up my build.
It seems very promising, but I have some troubles configuring it to exclude 
directories from the input files hash calculation.

I tried to configure `maven-build-cache-config.xml` with :

```
<global>
    <excludes>
        <exclude>path/to/exclude</exclude>
    </excludes>
</global>
```

I throught it would be possible to put a path relative to a module root, or to 
the root of the multimodule project but I could not make it work this way.
The only configuration that worked was by using an absolute path in the config 
file (which is not really portable).
I also tried using properties like ${maven.multiModuleProjectDirectory} to 
build the full path but it is not replaced during the build.

Then I found out about the `maven.build.cache.exclude` property prefix, so I 
got it to work by configuring the directories to ignore in the module pom, 
prefixing the directory with ${basedir}:

```
<properties>
<maven.build.cache.exclude.path1>${basedir}/path/to/exclude</maven.build.cache.exclude.path1>
</properties>
```

Is it supposed to be configured this way or is their an easier way to use a 
relative path ?

Thanks & Regards,
Vincent