bpo-35296: make install now installs the internal API (GH-10665)

make install now also installs the internal API: Include/internal/*.h
header files.

Makefile.pre.in

@@ -1446,11 +1446,21 @@ inclinstall:
 		else	true; \
 		fi; \
 	done
+	@if test ! -d $(DESTDIR)$(INCLUDEPY)/internal; then \
+		echo "Creating directory $(DESTDIR)$(INCLUDEPY)/internal"; \
+		$(INSTALL) -d -m $(DIRMODE) $(DESTDIR)$(INCLUDEPY)/internal; \
+	else	true; \
+	fi
 	@for i in $(srcdir)/Include/*.h; \
 	do \
 		echo $(INSTALL_DATA) $$i $(INCLUDEPY); \
 		$(INSTALL_DATA) $$i $(DESTDIR)$(INCLUDEPY); \
 	done
+	@for i in $(srcdir)/Include/internal/*.h; \
+	do \
+		echo $(INSTALL_DATA) $$i $(INCLUDEPY)/internal; \
+		$(INSTALL_DATA) $$i $(DESTDIR)$(INCLUDEPY)/internal; \
+	done
 	$(INSTALL_DATA) pyconfig.h $(DESTDIR)$(CONFINCLUDEPY)/pyconfig.h
 
 # Install the library and miscellaneous stuff needed for extending/embedding

Misc/NEWS.d/next/C API/2018-11-22-18-34-23.bpo-35296.nxrIQt.rst

+``make install`` now also installs the internal API:
+``Include/internal/*.h`` header files.