Multicorn: Build failure with PG12: src/multicorn.h:16:10: fatal error: nodes/relation.h: No such file or directory

Building against PG 12beta1:

17:57:48 creating build/temp.linux-amd64-2.7/src
17:57:48 x86_64-linux-gnu-gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fno-strict-aliasing -g -O2 -fdebug-prefix-map=/<<PKGBUILDDIR>>=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -I/usr/include/postgresql -I/usr/include/postgresql -I/usr/include/postgresql/12/server -I/usr/include/python2.7 -c src/utils.c -o build/temp.linux-amd64-2.7/src/utils.o -shared
17:57:48 In file included from /usr/include/postgresql/12/server/foreign/fdwapi.h:17,
17:57:48                  from src/multicorn.h:9,
17:57:48                  from src/utils.c:17:
17:57:48 /usr/include/postgresql/12/server/nodes/pathnodes.h:834:2: warning: function declaration isn't a prototype [-Wstrict-prototypes]
17:57:48   void  (*amcostestimate) (); /* AM's cost estimator */
17:57:48   ^~~~
17:57:48 In file included from src/utils.c:17:
17:57:48 src/multicorn.h:16:10: fatal error: nodes/relation.h: No such file or directory
17:57:48  #include "nodes/relation.h"
17:57:48           ^~~~~~~~~~~~~~~~~~
17:57:48 compilation terminated.

Is Multicorn still supported? Even building against 9.6 is still broken (#171). Packagers would very much appreciate if we didn’t have to carry a ton of patches.