Set "compatibility version" and "current version".

For #304.

Change-Id: I74e2720ecad9a0b22e8cd73dbffe2e02599827c5
Reviewed-on: https://code-review.googlesource.com/c/re2/+/58450
Reviewed-by: Paul Wankadia <junyer@google.com>

Makefile

diff --git a/Makefile b/Makefile
index 44cedee..ba5e4f6 100644
--- a/Makefile
+++ b/Makefile
@@ -55,7 +55,7 @@
 SOEXT=dylib
 SOEXTVER=$(SONAME).$(SOEXT)
 SOEXTVER00=$(SONAME).0.0.$(SOEXT)
-MAKE_SHARED_LIBRARY=$(CXX) -dynamiclib -Wl,-install_name,$(libdir)/libre2.$(SOEXTVER),-exported_symbols_list,libre2.symbols.darwin $(RE2_LDFLAGS) $(LDFLAGS)
+MAKE_SHARED_LIBRARY=$(CXX) -dynamiclib -Wl,-compatibility_version,$(SONAME),-current_version,$(SONAME).0.0,-install_name,$(libdir)/libre2.$(SOEXTVER),-exported_symbols_list,libre2.symbols.darwin $(RE2_LDFLAGS) $(LDFLAGS)
 else ifeq ($(shell uname),SunOS)
 SOEXT=so
 SOEXTVER=$(SOEXT).$(SONAME)