Make build more portable

[remkop]

Allow build to run even when biz.aQute.bnd.builder plugin is not available.

if (System.getenv('BINTRAY_USER')) {
    apply plugin: 'biz.aQute.bnd.builder'
} else {
    try { // only apply if available
        Class.forName('aQute.bnd.gradle.BndPlugin');
        Class.forName('aQute.bnd.build.Project');
        apply plugin: 'biz.aQute.bnd.builder'
    } catch (Throwable ignored) {}
}
pluginManager.withPlugin('biz.aQute.bnd.builder') {
    apply plugin: 'biz.aQute.bnd.builder'
    configurations {
        bundleCompile
        baseline
    }
    dependencies {
        baseline('group': group, 'name': jar.baseName, 'version': "(,${jar.version}[") {
        force = true
        transitive = false
      }
    }
    sourceSets {
        bundle
    }
    task bundle(type: Bundle) {
        from sourceSets.bundle.output
        bndfile = 'bnd.bnd'
        sourceSet = sourceSets.bundle
    }
}