gnu: mlt: Don't hardcode gcc value.

* gnu/packages/video.scm (mlt)[arguments]: Adjust make-flags to use
cc-for-target, cxx-for-target.

gnu/packages/video.scm

@@ -2998,7 +2998,8 @@ (define-public mlt
     (build-system gnu-build-system)
     (arguments
      `(#:tests? #f                      ; no tests
-       #:make-flags '("CC=gcc" "CXX=g++")
+       #:make-flags '(,(string-append "CC=" (cc-for-target))
+                      ,(string-append "CXX=" (cxx-for-target)))
        #:configure-flags
        (list "--enable-gpl3"
              "--enable-gpl")