GROOVY-8775, GROOVY-9197: Ant: separate JVM and compilation classpaths (Groovy 2.5 backport)

[eric-milles]

• updated docs for groovyc ant task
• add support for more joint compilation options
• add notes on joint compiler arguments and refactor minor stuff

<groovyc>
  <javac _attribute_ />
</groovyc>

• bootclasspath
• bootclasspathref
• deprecation
• modulepath
• modulepathref
• modulesourcepath
• modulesourcepathref
• nativeheaderdir
• nowarn
• release
• upgrademodulepath
• upgrademodulepathref

<groovyc>
  <javac>
    <compilerarg value="-proc:_option_" />
  </javac>
</groovyc>

<groovyc previewFeatures="true">
  <javac><!-- receives "--enable-preview" argument -->
</groovyc>

[daniellansun]

Merged. Thanks.