Specify the soname in the library and its name

This provides users of this library the guarantee that the ABI will
stay stable when MAJOR will reach 1, and will stay backwards compatible
for the entire duration of the 1.x.y branch.

It does require the maintainers to always update the version in the
Makefile at every ABI change.

Makefile

@@ -1,5 +1,9 @@
 #!/usr/bin/make -f
 
+MAJOR := 0
+MINOR := 1
+PATCH := 0
+VERSION := $(MAJOR).$(MINOR).$(PATCH)
 BUILD_DIR := build
 RELEASE_OPTIMIZE_FLAGS ?= -g -O3
 DEBUG_OPTIMIZE_FLAGS ?= -g -O0
@@ -9,8 +13,10 @@ CC = gcc
 EMCC = emcc
 AFL_CC = afl-gcc
 AFL_CXX = afl-g++
-RELEASE_TARGET := $(BUILD_DIR)/libolm.so
+RELEASE_TARGET := $(BUILD_DIR)/libolm.so.$(VERSION)
-DEBUG_TARGET := $(BUILD_DIR)/libolm_debug.so
+RELEASE_SYMLINKS := $(BUILD_DIR)/libolm.so.$(MAJOR) $(BUILD_DIR)/libolm.so
+DEBUG_TARGET := $(BUILD_DIR)/libolm_debug.so.$(VERSION)
+DEBUG_SYMLINKS := $(BUILD_DIR)/libolm_debug.so.$(MAJOR) $(BUILD_DIR)/libolm_debug.so
 JS_TARGET := javascript/olm.js
 
 JS_EXPORTED_FUNCTIONS := javascript/exported_functions.json
@@ -87,22 +93,30 @@ $(JS_TARGET): LDFLAGS += $(JS_OPTIMIZE_FLAGS)
 
 ### top-level targets
 
-lib: $(RELEASE_TARGET)
+lib: $(RELEASE_TARGET) $(RELEASE_SYMLINKS)
 .PHONY: lib
 
 $(RELEASE_TARGET): $(RELEASE_OBJECTS)
 	$(CXX) $(LDFLAGS) --shared -fPIC \
+            -Wl,-soname,libolm.so.$(MAJOR) \
             -Wl,--version-script,version_script.ver \
             $(OUTPUT_OPTION) $(RELEASE_OBJECTS)
 
-debug: $(DEBUG_TARGET)
+$(RELEASE_SYMLINKS):
+	ln -s libolm.so.$(VERSION) $@
+
+debug: $(DEBUG_TARGET) $(DEBUG_SYMLINKS)
 .PHONY: debug
 
 $(DEBUG_TARGET): $(DEBUG_OBJECTS)
 	$(CXX) $(LDFLAGS) --shared -fPIC \
+            -Wl,-soname,libolm_debug.so.$(MAJOR) \
             -Wl,--version-script,version_script.ver \
             $(OUTPUT_OPTION) $(DEBUG_OBJECTS)
 
+$(DEBUG_SYMLINKS):
+	ln -s libolm_debug.so.$(VERSION) $@
+
 js: $(JS_TARGET)
 .PHONY: js