This commit reverts the changes made in 7bc44a90 so that developers who
do not use the Gradle daemon are not adversely affected by the explicit
JVM args that were introduced in that commit.

Developers who wish to run the build against JDK 9 with the Gradle
daemon can add the following to the gradle.properties file in their
'gradle user home' directory (e.g., ~/.gradle/gradle.properties):

    org.gradle.daemon=true
    org.gradle.jvmargs=-XX:MaxMetaspaceSize=1024m -Xmx1024m

See also: https://issues.gradle.org/browse/GRADLE-3256

Issue: SPR-12549

This commit adds the following to gradle.properties in order to execute
the Gradle daemon on JDK 9, since Gradle's DaemonParameters
automatically sets the MaxPermSize JVM argument, which is no longer
supported on JDK 9.

org.gradle.jvmargs=-XX:MaxMetaspaceSize=1024m -Xmx1024m

Issue: SPR-12549

This reverts commit 4788875a.

See http://blog.springsource.org/2013/01/16/next-stop-spring-framework-4-0
for details.

The new jdiff task generates a report of API differences between the
current version (i.e. the value of `version` in gradle.properties) and
any older version of the framework, as specified by -DOLD_VERSION at
the command line, or defaulting to `previousVersion` in
gradle.properties.

Running the command requires a separate clone directory pinned to the
desired old version, as specified by -DOLD_VERSION_ROOT at the command
line. This creates challenges from a build automation perspective,
largely because Gradle doesn't (yet) have APIs for working with Git.

This task may be further automated and included in nightly CI runs, but
in the meantime, a number of reports back to 3.1.3.RELEASE have been
generated manually and uploaded to [1], where one can now find the
following entries in the directory listing:

 - 3.1.3.RELEASE_to_3.2.0.RC1
 - 3.2.0.M1_to_3.2.0.M2
 - 3.2.0.M2_to_3.2.0.RC1
 - 3.2.0.RC1_to_3.2.0.BUILD-SNAPSHOT

Ideally, the final entry there would be kept up-to-date on a daily
basis - again we may revisit doing so in the future. Going forward,
reports will be generated and uploaded manually on an as needed basis
and as part of the release process.

The goal of these reports are as follows:

 - to ease the process of ensuring backward compatibility

 - to aid in code reviews, particularly when reviewing large pull
   requests

 - to ease the process of creating migration guides for project
   maintainers, i.e. to help us remember what's changed

 - to allow ambitious end-users to discover what's been changing at the
   API level without without needing to wait for detailed "what's new in
   version X" and/or migration guide documentation

See documentation in jdiff.gradle for usage details.

Note that the jdiff-1.1.1 distribution as downloaded from [2] has been
added wholesale to the source tree under gradle/jdiff instead of
uploading JDiff jars to repo.springsource.org as we would normally do.
This is due to some unfortunate limitations in the implementation of the
jdiff ant task that require a phisical JDIFF_HOME directory. Checking in
the jars and various resources represents the simplest and most
pragmatic solution to this problem, though ambitious contributors are
free to do what's necessary to arrive at a more elegant arrangement.

[1]: http://static.springframework.org/spring-framework/docs
[2]: http://sourceforge.net/projects/javadiff/files/latest/download

Issue: SPR-9957