fix: build with Xcode 10 may fail for missing plugins...xcconfig file

lib/services/ios-project-service.ts

@@ -1301,6 +1301,13 @@ We will now place an empty obsolete compatability white screen LauncScreen.xib f
 			await this.mergeXcconfigFiles(appResourcesXcconfigPath, pluginsXcconfigFilePath);
 		}
 
+		if (!this.$fs.exists(pluginsXcconfigFilePath)) {
+			// We need the pluginsXcconfig file to exist in platforms dir as it is required in the native template:
+			// https://github.com/NativeScript/ios-runtime/blob/9c2b7b5f70b9bee8452b7a24aa6b646214c7d2be/build/project-template/__PROJECT_NAME__/build-debug.xcconfig#L3
+			// From Xcode 10 in case the file is missing, this include fails and the build itself fails (was a warning in previous Xcode versions).
+			this.$fs.writeFile(pluginsXcconfigFilePath, "");
+		}
+
 		// Set Entitlements Property to point to default file if not set explicitly by the user.
 		const entitlementsPropertyValue = this.$xCConfigService.readPropertyValue(pluginsXcconfigFilePath, constants.CODE_SIGN_ENTITLEMENTS);
 		if (entitlementsPropertyValue === null && this.$fs.exists(this.$iOSEntitlementsService.getPlatformsEntitlementsPath(projectData))) {

test/ios-project-service.ts

@@ -1029,4 +1029,18 @@ describe("Merge Project XCConfig files", () => {
 			assertPropertyValues(expected, destinationFilePath, testInjector);
 		}
 	});
+
+	it("creates empty plugins-<config>.xcconfig in case there are no build.xcconfig in App_Resources and in plugins", async () => {
+		// run merge for all release: debug|release
+		for (const release in [true, false]) {
+			await (<any>iOSProjectService).mergeProjectXcconfigFiles(release, projectData);
+
+			const destinationFilePath = release ? (<any>iOSProjectService).getPluginsReleaseXcconfigFilePath(projectData)
+				: (<any>iOSProjectService).getPluginsDebugXcconfigFilePath(projectData);
+
+			assert.isTrue(fs.exists(destinationFilePath), 'Target build xcconfig is missing for release: ' + release);
+			const content = fs.readFile(destinationFilePath).toString();
+			assert.equal(content, "");
+		}
+	});
 });