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(); }
<td class='sub'>Wojtek's customized Guix</td><td class='sub right'></td></tr></table>
<table class='tabs'><tr><td>
<a href='/guix/about/'>about</a><a href='/guix/'>summary</a><a href='/guix/refs/?id=88563b71c70ff42168771402b2820f01ccd02420'>refs</a><a class='active' href='/guix/log/tests/size.scm'>log</a><a href='/guix/tree/tests/size.scm?id=88563b71c70ff42168771402b2820f01ccd02420'>tree</a><a href='/guix/commit/tests/size.scm?id=88563b71c70ff42168771402b2820f01ccd02420'>commit</a><a href='/guix/diff/tests/size.scm?id=88563b71c70ff42168771402b2820f01ccd02420'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/tests/size.scm'>
<input type='hidden' name='id' value='88563b71c70ff42168771402b2820f01ccd02420'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=88563b71c70ff42168771402b2820f01ccd02420'>root</a>/<a href='/guix/log/tests?id=88563b71c70ff42168771402b2820f01ccd02420'>tests</a>/<a href='/guix/log/tests/size.scm?id=88563b71c70ff42168771402b2820f01ccd02420'>size.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/tests/size.scm?id=88563b71c70ff42168771402b2820f01ccd02420&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2018-11-12 23:37:13 +0100'>2018-11-12</span></td><td><a href='/guix/commit/tests/size.scm?id=9ed86fe175c15c819d6d86681c8136ff6bc927c0'>tests: Add 'test-assertm' to (guix tests).</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/tests.scm (test-assertm): New macro.
* tests/gexp.scm (test-assertm): Remove.
* tests/profiles.scm (test-assertm): Remove.
* tests/challenge.scm (%store, test-assertm): Remove.
* tests/debug-link.scm (%store, test-assertm): Remove.
* tests/size.scm (%store, test-assertm): Remove.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-04-07 00:17:16 +0200'>2017-04-07</span></td><td><a href='/guix/commit/tests/size.scm?id=c923f831160ddbda21d8ea1f2df4670550f3e9d8'>size: Add test to multiple 'store-profile' arguments.</a><span class='msg-avail'>...<span class='msg-tooltip'>* tests/size.scm ("store-profile with multiple items"): New test.
</span></span></td><td>Ludovic Courtès</td></tr>