build-system/ruby: Add #:gem-flags parameter.

* guix/build-system/ruby.scm (build): add 'gem-flags' key
* guix/build/ruby-build-system.scm (build): use 'gem-flags' key
* doc/guix.texi (Build Systems): Mention #:gem-flags.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -2458,7 +2458,8 @@ implements the RubyGems build procedure used by Ruby packages, which
 involves running @code{gem build} followed by @code{gem install}.
 
 Which Ruby package is used can be specified with the @code{#:ruby}
-parameter.
+parameter.  A list of additional flags to be passed to the @command{gem}
+command can be specified with the @code{#:gem-flags} parameter.
 @end defvr
 
 @defvr {Scheme Variable} waf-build-system

guix/build-system/ruby.scm

@@ -71,6 +71,7 @@
 
 (define* (ruby-build store name inputs
                      #:key
+                     (gem-flags ''())
                      (test-target "test")
                      (tests? #t)
                      (phases '(@ (guix build ruby-build-system)
@@ -95,6 +96,7 @@
                                (source
                                 source))
                    #:system ,system
+                   #:gem-flags ,gem-flags
                    #:test-target ,test-target
                    #:tests? ,tests?
                    #:phases ,phases

guix/build/ruby-build-system.scm

@@ -63,7 +63,8 @@ directory."
       (zero? (system* "rake" test-target))
       #t))
 
-(define* (install #:key source inputs outputs #:allow-other-keys)
+(define* (install #:key source inputs outputs (gem-flags '())
+                  #:allow-other-keys)
   (let* ((ruby-version
           (match:substring (string-match "ruby-(.*)\\.[0-9]$"
                                          (assoc-ref inputs "ruby"))
@@ -72,10 +73,11 @@ directory."
          (gem-home (string-append out "/lib/ruby/gems/" ruby-version ".0")))
     (setenv "GEM_HOME" gem-home)
     (mkdir-p gem-home)
-    (zero? (system* "gem" "install" "--local"
+    (zero? (apply system* "gem" "install" "--local"
                   (first-matching-file "\\.gem$")
                   ;; Executables should go into /bin, not /lib/ruby/gems.
-                    "--bindir" (string-append out "/bin")))))
+                  "--bindir" (string-append out "/bin")
+                  gem-flags))))
 
 (define %standard-phases
   (modify-phases gnu:%standard-phases