utils: Change 'wrap-program' to preserve the original argv[0].

Suggested by Mark H Weaver <mhw@netris.org>
in <http://bugs.gnu.org/19138>.

* guix/build/utils.scm (wrap-program): Change wrapper to use "exec -a
  PROG" instead of just "exec".

guix/build/utils.scm

@@ -762,7 +762,7 @@ contents:
   #!location/of/bin/bash
   export PATH=\"/gnu/.../bar/bin\"
   export CERT_PATH=\"$CERT_PATH${CERT_PATH:+:}/gnu/.../baz/certs:/qux/certs\"
-  exec location/of/.foo-real
+  exec -a location/of/foo location/of/.foo-real \"$@\"
 
 This is useful for scripts that expect particular programs to be in $PATH, for
 programs that expect particular shared libraries to be in $LD_LIBRARY_PATH, or
@@ -786,6 +786,7 @@ the previous wrapper."
           (copy-file prog prog-real)
           prog-real)
         (wrapper-file-name number)))
+
   (let* ((number   (next-wrapper-number))
          (target   (wrapper-target number))
          (wrapper  (wrapper-file-name (1+ number)))
@@ -815,10 +816,11 @@ the previous wrapper."
     (with-output-to-file prog-tmp
       (lambda ()
         (format #t
-                "#!~a~%~a~%exec \"~a\" \"$@\"~%"
+                "#!~a~%~a~%exec -a \"~a\" \"~a\" \"$@\"~%"
                 (which "bash")
                 (string-join (map export-variable vars)
                              "\n")
+                (canonicalize-path prog)
                 (canonicalize-path target))))
 
     (chmod prog-tmp #o755)