Build: ignore deprecation warnings

We currently fail on any deprecations found during the build. However,
this includes things deprecated within ES, which adds a heavy burden in
order to deprecate apis (requring to add suppressions to all internal
callers of the API).

This change adds `-deprecation` to xlint. We should consider in the
future having a task to "see" what deprecated apis we currently use for
analysis.

buildSrc/src/main/groovy/org/elasticsearch/gradle/BuildPlugin.groovy

@@ -378,7 +378,7 @@ class BuildPlugin implements Plugin<Project> {
                  * -serial because we don't use java serialization.
                  */
                 // don't even think about passing args with -J-xxx, oracle will ask you to submit a bug report :)
-                options.compilerArgs << '-Werror' << '-Xlint:all,-path,-serial,-options' << '-Xdoclint:all' << '-Xdoclint:-missing'
+                options.compilerArgs << '-Werror' << '-Xlint:all,-path,-serial,-options,-deprecation' << '-Xdoclint:all' << '-Xdoclint:-missing'
                 // compile with compact 3 profile by default
                 // NOTE: this is just a compile time check: does not replace testing with a compact3 JRE
                 if (project.compactProfile != 'full') {