https://bugs.gentoo.org/567332

this is a snippet taken from the upstream commit:
09eae589d3954a1e1206aa01108ee75ef57776da Used in-tree header file for c++ tests.

--- a/tests/Makefile.in
+++ b/tests/Makefile.in
@@ -35,6 +35,8 @@
 # WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 # PURPOSE.
 
+AM_CPPFLAGS = -I$(top_srcdir)/src
+
 VPATH = @srcdir@
 am__is_gnu_make = { \
   if test -z '$(MAKELEVEL)'; then \