Unnecessary eager check for GraalVM installation breaks builds when using other JDKs

[alvarosanchez]

#274 introduced an unnecessary eager check for a GraalVM installation:

https://github.com/graalvm/native-build-tools/blob/master/native-maven-plugin/src/main/java/org/graalvm/buildtools/maven/NativeExtension.java#L120-L124

This halts the build with the following exception:

[INFO] Caused by: org.apache.maven.plugin.MojoExecutionException: 'gu' tool wasn't found. This probably means that JDK at isn't a GraalVM distribution.
[INFO]     at org.graalvm.buildtools.Utils.getJavaHomeNativeImage (Utils.java:89)
[INFO]     at org.graalvm.buildtools.Utils.getNativeImage (Utils.java:121)
[INFO]     at org.graalvm.buildtools.maven.NativeExtension.afterProjectsRead (NativeExtension.java:121)

Because this is done so early in an extension, it breaks builds even if no NBT mojo is being invoked.

The check should only be done lazily right before the graalvmJava variable is needed

[snicoll]

A note that it should also be possible to invoke a Mojo of Native Build Tools that does not need GraalVM. add-reachability-metadata in particular.

[sdeleuze]

I start working on this and will submit a PR.

[sschuberth]

I just filed a similar issue on the Gradle build file level, see #350.