ThymeleafAutoConfiguration fails if thymeleaf-spring5 is not present

[koziolk]

This commit adds an extra check for the presence of thymeleaf-spring5
library on the classpath. Since now ThymeleafAutoConfiguration is
considered only if both thymeleaf-spring5 and thymeleaf jars are present.

gh-16079

[koziolk]

The current code base from master does not build.
When one run ./mvnw clean install it fails when executing Spring Boot AutoConfigure tests.

You can spot this build failure in the latest builds:

• https://ci.spring.io/teams/spring-boot/pipelines/spring-boot/jobs/build-pull-requests/builds/1877
• https://ci.spring.io/teams/spring-boot/pipelines/spring-boot/jobs/build-pull-requests/builds/1878
• https://ci.spring.io/teams/spring-boot/pipelines/spring-boot/jobs/build-pull-requests/builds/1880

I was able to reproduce it locally with the following configuration:

JVM/Maven:

Apache Maven 3.5.4 (1edded0938998edf8bf061f1ceb3cfdeccf443fe; 2018-06-17T20:33:14+02:00)
Maven home: /Users/koziolk/.m2/wrapper/dists/apache-maven-3.5.4-bin/4lcg54ki11c6mp435njk296gm5/apache-maven-3.5.4
Java version: 1.8.0_202, vendor: AdoptOpenJdk, runtime: /Library/Java/JavaVirtualMachines/adoptopenjdk-8.jdk/Contents/Home/jre
Default locale: en_PL, platform encoding: UTF-8
OS name: "mac os x", version: "10.14.4", arch: "x86_64", family: "mac"

Git revision
3e3353e0e977d2e229e2490a2e6f292b9fa50894

Let me know if it's a known case or if I should raise an issue for it.

[snicoll]

Thank you. As you’ve seen yourself CI has been failing, probably due to a change in a snapshot dependency. We are aware of it and will work on a fix.

[snicoll]

@koziolk thank you for making your first contribution to Spring Boot. Your work has been merged in 2.1.x and master.

[snicoll]

Also, @mbhave reminded me we should have requested a test for this. I had to first rewrite to use the new test utility in 29b1b38 and I've added a test in 6c3d64a.