add Makefile and CI support

Add a Makefile so that pEpEngineSequoiaBackend can be build using the
usual pEp style without directly invoking cargo, supporting the usual
targets build, install, uninstall, test, clean, all.
Make the project easy to install and uninstall, with a user-settable
prefix.
The Makefile is extensible with a local.conf file, of which we provide
an example.

Add CI support.

Add a .gitignore file.

Co-authored-by: Heck <heck@pep.foundation>
Co-authored-by: Neal H. Walfield <neal@walfield.org>
Co-authored-by: Luca Saiu <positron@pep.foundation>
Co-authored-by: Devan Carpenter <dvn@pep-project.org>
Co-authored-by: Volker Birk <volker.birk@pep-project.org>

.gitignore

@@ -1,2 +1,3 @@
 *~
 /target
+/local.conf

.gitlab-ci.yml

@@ -60,6 +60,24 @@ bookworm:
     CARGO_HOME: /cargo
   after_script: []
 
+makefile:
+  tags:
+    - linux
+  stage: build
+  image: registry.gitlab.com/sequoia-pgp/build-docker-image/bookworm-prebuild:latest
+  needs:
+    - codespell
+  script:
+    - make build
+    - make test
+    - make clean
+    - DEBUG=debug make test
+  variables:
+    BUILD: /target
+    CARGO_HOME: /cargo
+    CARGO_TARGET_DIR:
+  after_script: []
+
 all_commits:
   # Test each commit up to main, to facilitate bisecting.
   stage: test

Makefile

@@ -0,0 +1,86 @@
+# Copyright 2023, pEp Foundation
+# This file is part of pEpEngineSequoiaBackend
+# This file may be used under the terms of the GNU General Public License version 3
+# see COPYING
+
+# Build config defaults
+DEBUG?=debug
+PREFIX?=/usr/local
+CARGO?=cargo
+BUILD?=_build
+# Build config overrides
+-include ./local.conf
+
+# Make sure CARGO_TARGET_DIR is not set by the user -- it would be ignored.
+CARGO_TARGET_DIR?=
+ifneq ($(CARGO_TARGET_DIR),)
+    $(error "the user should not set 'CARGO_TARGET_DIR': 'BUILD' can be used \
+             instead.")
+endif
+
+#export as env var to sub-shells
+export CARGO_TARGET_DIR=$(BUILD)
+
+# constants
+LIB_NAME=libpep_engine_sequoia_backend
+
+# determine library file extension
+PLATFORM:=$(shell uname)
+ifeq ($(PLATFORM),Linux)
+    DYNLIB_EXT=so
+else ifeq ($(PLATFORM),Darwin)
+    DYNLIB_EXT=dylib
+else
+    $(error "Dont know how to build for '$(PLATFORM)'")
+endif
+
+# DEBUG can be defined as "release", "debug" or "maintainer".
+ifeq ($(DEBUG),maintainer)  # For compatibility with other pEp repos.
+    VARIANT_NAME=debug
+    VARIANT_FLAGS=
+else ifeq ($(DEBUG),debug)
+    VARIANT_NAME=debug
+    VARIANT_FLAGS=
+else ifeq ($(DEBUG),release)
+    VARIANT_NAME=release
+    VARIANT_FLAGS=--release
+else
+    $(error "build option 'DEBUG' must be 'release', 'debug' or 'maintainer'")
+endif
+
+LIB_DYNAMIC_PATH=$(CARGO_TARGET_DIR)/$(VARIANT_NAME)/$(LIB_NAME).$(DYNLIB_EXT)
+LIB_STATIC_PATH=$(CARGO_TARGET_DIR)/$(VARIANT_NAME)/$(LIB_NAME).a
+PKGCONFIG_PATH=$(CARGO_TARGET_DIR)/$(VARIANT_NAME)/pep_engine_sequoia_backend.pc
+LIB_DIR=$(PREFIX)/lib/
+PKGCONFIG_DIR=$(PREFIX)/share/pkgconfig/
+
+CARGO_FLAGS+=$(VARIANT_FLAGS)
+
+ifneq ($(filter Darwin %BSD,$(shell uname -s)),)
+    INSTALL?=ginstall
+else
+    INSTALL?=install
+endif
+
+.PHONY: all build install uninstall test clean
+all: build
+
+build:
+	$(CARGO) build $(CARGO_FLAGS)
+
+install: build
+	mkdir -p $(LIB_DIR) $(PKGCONFIG_DIR)
+	$(INSTALL) $(LIB_DYNAMIC_PATH) $(LIB_DIR)
+	$(INSTALL) $(LIB_STATIC_PATH) $(LIB_DIR)
+	$(INSTALL) $(PKGCONFIG_PATH) $(PKGCONFIG_DIR)
+
+uninstall:
+	rm -f $(LIB_DIR)/$(LIB_NAME).$(DYNLIB_EXT)
+	rm -f $(LIB_DIR)/$(LIB_NAME).a
+	rm -f $(PKGCONFIG_DIR)/pep_engine_sequoia_backend.pc
+
+test:
+	$(CARGO) test $(CARGO_FLAGS)
+
+clean:
+	$(CARGO) clean

local.conf.example

@@ -0,0 +1,17 @@
+# This is an Example build config (local.conf)
+# The values here reflect the defaults.
+# If needed, copy this file into a file called
+# `local.conf` and adjust the values
+
+# The dir where the build artifacts get installed into
+# PREFIX=/usr/local
+
+# Build variant: one of release , debug , maintainer (maintainer exists for
+# compatibility with other pEp projects: in this project maintainer and debug
+# builds are the same).  The default is debug .
+# DEBUG=release
+
+# Defines the build dir.  This can be a relative or an absolute path.
+# BUILD=_build
+
+