https://bugs.gentoo.org/704044#c8

Patch by Alexey

--- a/src/Makefile.am	2020-04-26 11:16:22.354103351 +0100
+++ b/src/Makefile.am	2020-04-26 11:16:31.584080899 +0100
@@ -18,8 +18,7 @@ LDFLAGS += -version-info $(B2_LIBRARY_VE
 lib_LTLIBRARIES = libb2.la
 libb2_la_LIBADD = # -lgomp -lpthread
 libb2_la_CPPFLAGS =  -DSUFFIX=  \
-                     $(LTDLINCL) \
-                     ${top_builddir}/src/
+                     $(LTDLINCL)
 
 include_HEADERS = blake2.h