lib/dns/gen.c:29:26: fatal error: isc/platform.h: No such file or directory

In OpenIndiana (an illumos distribution) we build BIND out-of-tree and BIND 9.14.9 fails to build for me with:

making all in /userland/ws/oi-userland/components/network/bind/build/amd64/lib/dns
make[3]: Entering directory '/userland/ws/oi-userland/components/network/bind/build/amd64/lib/dns'
/usr/gcc/6/bin/gcc -m64 -O3  -D_XOPEN_SOURCE=600 -D__EXTENSIONS__=1 -D_XPG6 -D_POSIX_PTHREAD_SEMANTICS -pthread  -I/usr/include/libxml2   -fPIC -I/userland/ws/oi-userland/components/network/bind/bind-9.14.9/lib/isc/include \
-D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64  \
 -m64 -Wl,-ztext -o gen /userland/ws/oi-userland/components/network/bind/bind-9.14.9/lib/dns/gen.c \
-lnsl -lsocket -lscf -ljson-c -llmdb   -lxml2 -lz -llzma -lpthread -lm -lsocket -lnsl  
/userland/ws/oi-userland/components/network/bind/bind-9.14.9/lib/dns/gen.c:29:26: fatal error: isc/platform.h: No such file or directory
 #include <isc/platform.h>
                          ^

The culprit seems to originated in d10fbdec.

I was able to make the problem away with the following patch:

--- bind-9.14.9/lib/dns/Makefile.in	2019-12-12 07:12:17.000000000 +0000
+++ bind-9.14.9/lib/dns/Makefile.in	2019-12-22 15:48:17.354544762 +0000
@@ -172,7 +172,7 @@ code.h:	gen
 	./gen -s ${srcdir} > code.h || { rm -f $@ ; exit 1; }
 
 gen: gen.c
-	${BUILD_CC} ${BUILD_CFLAGS} -I${top_srcdir}/lib/isc/include \
+	${BUILD_CC} ${BUILD_CFLAGS} -I../../lib/isc/include \
 	${LFS_CFLAGS} ${LFS_LDFLAGS} \
 	${BUILD_CPPFLAGS} ${BUILD_LDFLAGS} -o $@ ${srcdir}/gen.c \
 	${BUILD_LIBS} ${LFS_LIBS}

If it's indeed an out-of-tree build issue, respective CI job might we worth.