gnu: agg: Fix build.

* gnu/packages/patches/agg-2.5-gcc8.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/graphics.scm (agg)[source]: Use it.

gnu/local.mk

@@ -836,6 +836,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/aegisub-boost68.patch			\
   %D%/packages/patches/aegisub-make43.patch			\
   %D%/packages/patches/agg-am_c_prototype.patch			\
+  %D%/packages/patches/agg-2.5-gcc8.patch			\
   %D%/packages/patches/akonadi-paths.patch		\
   %D%/packages/patches/akonadi-not-relocatable.patch		\
   %D%/packages/patches/akonadi-timestamps.patch		\

gnu/packages/graphics.scm

@@ -1539,7 +1539,8 @@ and understanding different BRDFs (and other component functions).")
                                         version ".tar.gz")))
               (sha256
                (base32 "07wii4i824vy9qsvjsgqxppgqmfdxq0xa87i5yk53fijriadq7mb"))
-              (patches (search-patches "agg-am_c_prototype.patch"))))
+              (patches (search-patches "agg-am_c_prototype.patch"
+                                       "agg-2.5-gcc8.patch"))))
     (build-system gnu-build-system)
     (arguments
      '(#:configure-flags

gnu/packages/patches/agg-2.5-gcc8.patch

@@ -0,0 +1,14 @@
+Taken from: https://gitweb.gentoo.org/repo/gentoo.git/tree/x11-libs/agg/files/agg-2.5-gcc8.patch.
+
+diff -uprN agg-2.5/include/agg_renderer_outline_aa.h agg-2.5-fixed/include/agg_renderer_outline_aa.h
+--- agg-2.5/include/agg_renderer_outline_aa.h	2018-08-25 02:02:52.507254241 +0800
++++ agg-2.5-fixed/include/agg_renderer_outline_aa.h	2018-08-25 02:04:15.291175316 +0800
+@@ -1375,7 +1375,7 @@ namespace agg
+         //---------------------------------------------------------------------
+         void profile(const line_profile_aa& prof) { m_profile = &prof; }
+         const line_profile_aa& profile() const { return *m_profile; }
+-        line_profile_aa& profile() { return *m_profile; }
++        const line_profile_aa& profile() { return *m_profile; }
+ 
+         //---------------------------------------------------------------------
+         int subpixel_width() const { return m_profile->subpixel_width(); }