pom: set release 8 on jdk9+

This fixes having Java 9+ API dependencies generated by the compiler
when building on JDK9+ despite targeting Java 8 bytecode. This fixes the
gpu plugin linking errors when running on Java 8 after being compiled on
9+.

pom.xml

@@ -38,6 +38,7 @@
 	<properties>
 		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
 		<java.version>1.8</java.version>
+		<java.release>8</java.release>
 		<lombok.version>1.18.4</lombok.version>
 
 		<maven.javadoc.skip>true</maven.javadoc.skip>
@@ -151,6 +152,25 @@
 		</dependencies>
 	</dependencyManagement>
 
+	<profiles>
+		<profile>
+			<id>java9</id>
+			<activation>
+				<jdk>[1.9,)</jdk>
+			</activation>
+			<build>
+				<plugins>
+					<plugin>
+						<artifactId>maven-compiler-plugin</artifactId>
+						<configuration>
+							<release>${java.release}</release>
+						</configuration>
+					</plugin>
+				</plugins>
+			</build>
+		</profile>
+	</profiles>
+
 	<build>
 		<plugins>
 			<plugin>