gnu: altermime: Use G-expressions.

* gnu/packages/mail.scm (altermime)[arguments]:
Rewrite as G-expressions.  Don't explicitly return #t from phases.

gnu/packages/mail.scm

@@ -837,11 +837,13 @@ Extension (MIME).")
                 "15zxg6spcmd35r6xbidq2fgcg2nzyv1sbbqds08lzll70mqx4pj7"))))
     (build-system gnu-build-system)
     (arguments
-     `(#:make-flags (list "CC=gcc"
-                          (string-append "PREFIX=" (assoc-ref %outputs "out")))
+     (list
+      #:make-flags
+      #~(list "CC=gcc"
+              (string-append "PREFIX=" #$output))
       #:tests? #f                       ; there are none
       #:phases
-       (modify-phases %standard-phases
+      #~(modify-phases %standard-phases
           (delete 'configure)
           (add-after 'unpack 'fix-bugs
             (lambda _
@@ -855,19 +857,16 @@ Extension (MIME).")
                  "if ((st->start)&&(*st->start != '\\0'))"))
               (substitute* "qpe.c"
                 (("if \\(lineend != '\\\\0'\\)")
-                "if (*lineend != '\\0')"))
-             #t))
+                 "if (*lineend != '\\0')"))))
           (add-after 'unpack 'install-to-prefix
             (lambda _
               (substitute* "Makefile"
                 (("/usr/local") "${PREFIX}")
-               (("cp altermime.*") "install -D -t ${PREFIX}/bin altermime\n"))
-             #t))
+                (("cp altermime.*") "install -D -t ${PREFIX}/bin altermime\n"))))
           (add-after 'unpack 'disable-Werror
             (lambda _
               (substitute* "Makefile"
-               (("-Werror") ""))
-             #t)))))
+                (("-Werror") "")))))))
     (home-page "https://pldaniels.com/altermime/")
     (synopsis "Modify MIME-encoded messages")
     (description