building_with_ant

1) Go to: /android-sdk-dir/tools/ant/build.xml and open it for editing.

2) Change:

<property name="java.target" value="1.x" />
<property name="java.source" value="1.x" />

to:

<property name="java.target" value="1.7" />
<property name="java.source" value="1.7" />


3) Rename <macrodef name="dex-helper"> to something like <macrodef name="_dex-helper"> so it uses 
the modified one in #4. Commenting it out will probably also work fine.

4) Override dex-helper with this modified macrodef (the important lines are --core-library and --no-strict):

    <macrodef name="dex-helper">
       <element name="external-libs" optional="yes" />
       <element name="extra-parameters" optional="yes" />
       <sequential>
            <!-- sets the primary input for dex. If a pre-dex task sets it to
                 something else this has no effect -->
            <property name="out.dex.input.absolute.dir" value="${out.classes.absolute.dir}" />
            <!-- set the secondary dx input: the project (and library) jar files
                 If a pre-dex task sets it to something else this has no effect -->
            <if>
                <condition>
                    <isreference refid="out.dex.jar.input.ref" />
                </condition>
                <else>
                    <path id="out.dex.jar.input.ref">
                        <path refid="project.all.jars.path" />
                    </path>
                </else>
            </if>
         <apply executable="${dx}" failonerror="true" parallel="true">
             <arg value="--dex" />
             <arg value="--core-library" />
             <arg value="--no-locals" />
             <arg value="--verbose" />
             <arg value="--output=${intermediate.dex.file}" />
             <arg value="--no-strict" />
             <extra-parameters />
             <arg path="${out.dex.input.absolute.dir}" />
             <path refid="out.dex.jar.input.ref" />
             <external-libs />
         </apply>
       </sequential>
    </macrodef>

5) add the following path above the <javac encoding="${java.encoding}" tag:

<path id="project.bootpath">
   <pathelement location="libs/core.jar" />
   <path refid="project.target.class.path" />
</path>

This will point to your project library so you can use the modded files as a jar. 
Make sure to name your jar file whatever you named it above (core.jar for this example). 
This file will go in your project_dir/libs directory.

6) Inside of <javac encoding="${java.encoding}", change 

bootclasspathref="project.javac.classpath"

to: bootclasspathref="project.bootpath"


7) Save the file and run ant as normal.