1. Do not force some compilation flags.
2. Set library version, respecting OpenBSD policy.
3. Avoid duplication of defines, linked to (2).
4. Do not use $(INSTALL_*) at the build stage.

Index: Makefile
--- Makefile.orig
+++ Makefile
@@ -38,7 +38,7 @@ ifneq (,$(findstring yes,$(CAPSTONE_DIET)))
 CFLAGS ?= -Os
 CFLAGS += -DCAPSTONE_DIET
 else
-CFLAGS ?= -O3
+CFLAGS ?= 
 endif
 
 ifneq (,$(findstring yes,$(CAPSTONE_X86_ATT_DISABLE)))
@@ -100,8 +100,11 @@ INSTALL_DATA ?= $(INSTALL_BIN) -m0644
 INSTALL_LIB ?= $(INSTALL_BIN) -m0755
 
 LIBNAME = capstone
+LIBVER ?= 0.0.0
+ifdef LIB$(LIBNAME)_VERSION
+LIBVER = $(LIB$(LIBNAME)_VERSION)
+endif
 
-
 DEP_ARM =
 DEP_ARM += $(wildcard arch/ARM/ARM*.inc)
 
@@ -320,7 +323,7 @@ else
 EXT = so
 VERSION_EXT = $(EXT).$(API_MAJOR)
 AR_EXT = a
-$(LIBNAME)_LDFLAGS += -Wl,-soname,lib$(LIBNAME).$(VERSION_EXT)
+$(LIBNAME)_LDFLAGS += -Wl,-soname,lib$(LIBNAME).$(EXT).$(LIBVER)
 endif
 endif
 endif
@@ -331,7 +334,7 @@ LIBRARY = $(BLDIR)/$(LIBNAME).$(VERSION_EXT)
 else ifeq ($(IS_CYGWIN),1)
 LIBRARY = $(BLDIR)/$(LIBNAME).$(VERSION_EXT)
 else	# *nix
-LIBRARY = $(BLDIR)/lib$(LIBNAME).$(VERSION_EXT)
+LIBRARY = $(BLDIR)/lib$(LIBNAME).$(EXT).$(LIBVER)
 CFLAGS += -fvisibility=hidden
 endif
 endif
@@ -501,10 +504,6 @@ endif
 ifeq ($(CAPSTONE_SHARED),yes)
 define install-library
 	$(INSTALL_LIB) $(LIBRARY) $1
-	$(if $(VERSION_EXT),
-		cd $1 && \
-		rm -f lib$(LIBNAME).$(EXT) && \
-		ln -s lib$(LIBNAME).$(VERSION_EXT) lib$(LIBNAME).$(EXT))
 endef
 else
 define install-library