gnu: arc-theme: Fix build.

* gnu/packages/web.scm (sassc/libsass-3.5): New public variable.
* gnu/packages/gnome.scm (arc-theme)[native-inputs]: Use it.

gnu/packages/gnome.scm

@@ -8290,7 +8290,7 @@ (define-public arc-theme
        ("inkscape" ,inkscape)
        ("optipng" ,optipng)
        ("pkg-config" ,pkg-config)
-       ("sassc" ,sassc)))
+       ("sassc" ,sassc/libsass-3.5)))
     (synopsis "A flat GTK+ theme with transparent elements")
     (description "Arc is a flat theme with transparent elements for GTK 3, GTK
 2, and GNOME Shell which supports GTK 3 and GTK 2 based desktop environments

gnu/packages/web.scm

@@ -1516,6 +1516,7 @@ (define-public serf
 (define-public libsass
   (package
     (name "libsass")
+    ;; When updating, check whether sassc/libsass-3.5 is still needed.
     (version "3.6.3")
     (source (origin
               (method git-fetch)
@@ -1595,6 +1596,29 @@ (define-public sassc
     (home-page "https://sass-lang.com/libsass")
     (license license:expat)))
 
+(define-public sassc/libsass-3.5
+  ;; Newer libsass versions suffor from a memory leak when building (some?)
+  ;; GTK themes <https://github.com/sass/libsass/issues/3033>.
+  (package
+    (inherit sassc)
+    (name "sassc")
+    (inputs
+     `(("libsass" ,
+        (package
+          (inherit libsass)
+          (name "libsass")
+          (version "3.5.5")
+          (source
+           (origin
+             (method git-fetch)
+             (uri (git-reference
+                   (url "https://github.com/sass/libsass.git")
+                   (commit version)))
+             (file-name (git-file-name name version))
+             (sha256
+              (base32
+               "0830pjcvhzxh6yixj82x5k5r1xnadjqzi16kp53213icbly0r9ma"))))))))))
+
 
 (define-public perl-apache-logformat-compiler
   (package