Thanks-to: Chris Mayo
https://bugs.gentoo.org/533774

--- a/configure	2021-01-21 10:14:17.000000000 +0100
+++ b/configure	2021-01-23 14:24:06.178883282 +0100
@@ -937,7 +937,7 @@
 runstatedir='${localstatedir}/run'
 includedir='${prefix}/include'
 oldincludedir='/usr/include'
-docdir='${datarootdir}/doc/${PACKAGE_TARNAME}'
+docdir='${docdir}'
 infodir='${datarootdir}/info'
 htmldir='${docdir}'
 dvidir='${docdir}'