gnu: fbcat, slepc: Use INVOKE.

* gnu/packages/linux.scm (fbcat)[arguments]: Substitute INVOKE for
SYSTEM*.
* gnu/packages/maths.scm (slepc)[arguments]: Likewise.

gnu/packages/linux.scm

@@ -4587,8 +4587,9 @@ (define-public fbcat
          (delete 'configure)            ; no configure script
          (add-after 'build 'build-documentation
            (lambda* (#:key make-flags #:allow-other-keys)
-             (zero? (apply system* "make" "-C" "doc"
-                           make-flags))))
+             (apply invoke "make" "-C" "doc"
+                    make-flags)
+             #t))
          (add-after 'build 'qualify-references
            (lambda* (#:key inputs outputs #:allow-other-keys)
              (let* ((pnmtopng (assoc-ref inputs "pnmtopng"))

gnu/packages/maths.scm

@@ -1647,7 +1647,7 @@ (define-public slepc
               (format #t "configure flags: ~s~%" flags)
               (setenv "SLEPC_DIR" (getcwd))
               (setenv "PETSC_DIR" (assoc-ref inputs "petsc"))
-              (zero? (apply system* "./configure" flags)))))
+              (invoke "./configure" flags))))
          (add-after 'install 'delete-doc
           ;; TODO: SLEPc installs HTML documentation alongside headers in
           ;; $out/include.  We'd like to move them to share/doc, but delete