guix build: Augment %PATCH-PATH when -L is passed.

Fixes <http://bugs.gnu.org/22259>.
Reported by Nikolay Merinov <nikolay.merinov@member.fsf.org>.

* guix/scripts/build.scm (%standard-build-options) <-L>: Augment
%PATCH-PATH.
* tests/guix-package.sh: Add test.

guix/scripts/build.scm

@@ -204,6 +204,7 @@ options handled by 'set-build-options-from-command-line', and listed in
                 (lambda (opt name arg result . rest)
                   ;; XXX: Imperatively modify the search paths.
                   (%package-module-path (cons arg (%package-module-path)))
+                  (%patch-path (cons arg (%patch-path)))
                   (set! %load-path (cons arg %load-path))
                   (set! %load-compiled-path (cons arg %load-compiled-path))
 

tests/guix-package.sh

@@ -225,6 +225,10 @@ cat > "$module_dir/foo.scm"<<EOF
 EOF
 guix package -i emacs-foo-bar-patched -n
 
+# Same when -L is used.
+( unset GUIX_PACKAGE_PATH;					\
+  guix package -L "$module_dir" -i emacs-foo-bar-patched -n )
+
 # Make sure installing from a file works.
 cat > "$module_dir/package.scm"<<EOF
 (use-modules (gnu))