add build of pEpEngineSequoiaBackend to the Makefile in case OPENPGP is SEQUOIA

Makefile

@@ -4,6 +4,8 @@
 # see LICENSE.txt
 
 HERE_REL := $(notdir $(CURDIR))
+SEQUOIA_BACKEND_PATH?=../pEpEngineSequoiaBackend
+CARGO_TARGET_DIR?=/tmp/pEp_engine_sequoia_backend
 
 include Makefile.conf
 
@@ -28,7 +30,13 @@ endif
 .PHONY: all $(BUILT_IN_MIME) codegen asn1 build install dbinstall uninstall clean tags test package db doc
 
 build: $(BUILT_IN_MIME) asn1
+ifeq ($(OPENPGP),SEQUOIA)
+	mkdir -p $(CARGO_TARGET_DIR)
+	cd $(SEQUOIA_BACKEND_PATH) && cargo build
+	PKG_CONFIG_PATH=$(CARGO_TARGET_DIR)/debug${PKG_CONFIG_PATH+:$PKG_CONFIG_PATH} $(MAKE) -C src
+else
 	$(MAKE) -C src
+endif
 
 all: build
 # `make all` is not for tests, that's what `make test` is for