We don't build/install interfaces by upstream's implicit rules.
This patch is used on two places; postgresql.spec and libecpg.spec -- keep those
in sync!
Related: rhbz#1618698
diff -ur postgresql-12.2/src/Makefile postgresql-12.2_patch/src/Makefile
--- postgresql-12.2/src/Makefile	2020-02-10 23:14:51.000000000 +0100
+++ postgresql-12.2_patch/src/Makefile	2020-03-02 12:49:45.530666894 +0100
@@ -20,7 +20,6 @@
 	backend/utils/mb/conversion_procs \
 	backend/snowball \
 	include \
-	interfaces \
 	backend/replication/libpqwalreceiver \
 	backend/replication/pgoutput \
 	fe_utils \
diff -ur postgresql-12.2/src/Makefile.global.in postgresql-12.2_patch/src/Makefile.global.in
--- postgresql-12.2/src/Makefile.global.in	2020-02-10 23:14:51.000000000 +0100
+++ postgresql-12.2_patch/src/Makefile.global.in	2020-03-02 12:47:40.970583609 +0100
@@ -550,7 +550,7 @@
 # How to link to libpq.  (This macro may be used as-is by backend extensions.
 # Client-side code should go through libpq_pgport or libpq_pgport_shlib,
 # instead.)
-libpq = -L$(libpq_builddir) -lpq
+libpq = -lpq
 
 # libpq_pgport is for use by client executables (not libraries) that use libpq.
 # We force clients to pull symbols from the non-shared libraries libpgport
@@ -580,7 +580,6 @@
 # Commonly used submake targets
 
 submake-libpq: | submake-generated-headers
-	$(MAKE) -C $(libpq_builddir) all
 
 submake-libpgport: | submake-generated-headers
 	$(MAKE) -C $(top_builddir)/src/port all