utils: `set-path-environment-variable' calls `unsetenv' for empty values.

* guix/build/utils.scm (set-path-environment-variable): When VALUE is
  the empty string, call `unsetenv' instead of `setenv'.
* gnu/packages/guile.scm (guile-2.0)[arguments]: Remove `unsetenv'
  trick.

gnu/packages/guile.scm

@@ -142,17 +142,6 @@ (define-public guile-2.0
     `(#:phases (alist-cons-before
                 'configure 'pre-configure
                 (lambda* (#:key inputs #:allow-other-keys)
-                  ;; By default we end up with GUILE_LOAD_PATH="" and
-                  ;; GUILE_LOAD_COMPILED_PATH="".  But that is equivalent to
-                  ;; ".", and breaks the build system when cross-compiling.
-                  ;; Thus, make sure they are unset.
-                  ;; TODO: Eventually fix `set-path-environment-variable'
-                  ;; for that case.
-                  ,@(if (%current-target-system)
-                        '((unsetenv "GUILE_LOAD_PATH")
-                          (unsetenv "GUILE_LOAD_COMPILED_PATH"))
-                        '())
-
                   ;; Tell (ice-9 popen) the file name of Bash.
                   (let ((bash (assoc-ref inputs "bash")))
                     (substitute* "module/ice-9/popen.scm"

guix/build/utils.scm

@@ -248,9 +248,17 @@ (define* (set-path-environment-variable env-var sub-directories input-dirs
 "
   (let* ((path  (search-path-as-list sub-directories input-dirs))
          (value (list->search-path-as-string path separator)))
-   (setenv env-var value)
-   (format #t "environment variable `~a' set to `~a'~%"
-           env-var value)))
+    (if (string-null? value)
+        (begin
+          ;; Never set ENV-VAR to an empty string because often, the empty
+          ;; string is equivalent to ".".  This is the case for
+          ;; GUILE_LOAD_PATH in Guile 2.0, for instance.
+          (unsetenv env-var)
+          (format #t "environment variable `~a' unset~%" env-var))
+        (begin
+          (setenv env-var value)
+          (format #t "environment variable `~a' set to `~a'~%"
+                  env-var value)))))
 
 (define (which program)
   "Return the complete file name for PROGRAM as found in $PATH, or #f if