Avoid use of absolute path where BND can't handle it

Fixes #734

gradle/manifest-gen.gradle

@@ -23,13 +23,13 @@ jar.bnd (
 	'Bundle-Vendor': 'Eclipse LSP4J',
 	'Bundle-RequiredExecutionEnvironment': 'JavaSE-11',
 	"-exportcontents": "org.eclipse.lsp4j.*",
-	"-savemanifest": "$buildDir/tmp/bnd/MANIFEST.MF",
+	"-savemanifest": "build/tmp/bnd/MANIFEST.MF",
 )
 
 //------------------------------------------------------
 // Generate a manifest for the source bundle
 
-def sourcesManifestFile = "$buildDir/tmp/sourcesJar/MANIFEST.MF"
+def sourcesManifestFile = "build/tmp/sourcesJar/MANIFEST.MF"
 
 task genSourcesManifest {
 	outputs.file(sourcesManifestFile)
@@ -64,7 +64,7 @@ sourcesJar {
 
 task eclipseManifest(type: Copy) {
 	dependsOn(jar)
-    from "$buildDir/tmp/bnd/MANIFEST.MF"
+    from "build/tmp/bnd/MANIFEST.MF"
     into 'META-INF'
 }