gnu: xtl: Use G-expressions.

* gnu/packages/cpp.scm (xtl)[arguments]:
Rewrite as G-expressions.

gnu/packages/cpp.scm

@@ -691,12 +691,13 @@ intuitive syntax and trivial integration.")
     (native-inputs
      (list doctest googletest nlohmann-json))
     (arguments
-     '(#:configure-flags
-       '("-DBUILD_TESTS=ON")
+     (list
+      #:configure-flags
+      #~(list "-DBUILD_TESTS=ON")
       #:phases
-       (modify-phases %standard-phases
+      #~(modify-phases %standard-phases
           (replace 'check
-           (lambda* _
+            (lambda _
               (with-directory-excursion "test"
                 (invoke "./test_xtl")))))))
     (home-page "https://github.com/QuantStack/xtl")