Add an install and an install-debug targets

This makes the user able to use the familiar `make install` syntax, and
allows overriding of the default directories using the DESTDIR and
PREFIX variables, for example:
 make DESTDIR=packaging PREFIX=/usr install

Makefile

@@ -4,6 +4,7 @@ MAJOR := 0
 MINOR := 1
 PATCH := 0
 VERSION := $(MAJOR).$(MINOR).$(PATCH)
+PREFIX ?= /usr/local
 BUILD_DIR := build
 RELEASE_OPTIMIZE_FLAGS ?= -g -O3
 DEBUG_OPTIMIZE_FLAGS ?= -g -O0
@@ -145,6 +146,22 @@ $(JS_EXPORTED_FUNCTIONS): $(PUBLIC_HEADERS)
 all: test js lib debug
 .PHONY: all
 
+install-debug: debug
+	test -d $(DESTDIR)$(PREFIX) || mkdir -p $(DESTDIR)$(PREFIX)
+	test -d $(DESTDIR)$(PREFIX)/lib || mkdir $(DESTDIR)$(PREFIX)/lib
+	install -Dm755 $(DEBUG_TARGET) $(DESTDIR)$(PREFIX)/lib/libolm_debug.so.$(VERSION)
+	ln -s libolm_debug.so.$(VERSION) $(DESTDIR)$(PREFIX)/lib/libolm_debug.so.$(MAJOR)
+	ln -s libolm_debug.so.$(VERSION) $(DESTDIR)$(PREFIX)/lib/libolm_debug.so
+.PHONY: install-debug
+
+install: lib
+	test -d $(DESTDIR)$(PREFIX) || mkdir -p $(DESTDIR)$(PREFIX)
+	test -d $(DESTDIR)$(PREFIX)/lib || mkdir $(DESTDIR)$(PREFIX)/lib
+	install -Dm755 $(RELEASE_TARGET) $(DESTDIR)$(PREFIX)/lib/libolm.so.$(VERSION)
+	ln -s libolm.so.$(VERSION) $(DESTDIR)$(PREFIX)/lib/libolm.so.$(MAJOR)
+	ln -s libolm.so.$(VERSION) $(DESTDIR)$(PREFIX)/lib/libolm.so
+.PHONY: install
+
 clean:;
 	rm -rf $(BUILD_DIR)
 .PHONY: clean