Makefile: set install_name on Darwin

https://bugs.gentoo.org/show_bug.cgi?id=546098
https://github.com/nodejs/http-parser/issues/356

--- http-parser-2.6.2/Makefile
+++ http-parser-2.6.2/Makefile
@@ -62,6 +62,8 @@
 ifneq (darwin,$(PLATFORM))
 # TODO(bnoordhuis) The native SunOS linker expects -h rather than -soname...
 LDFLAGS_LIB += -Wl,-soname=$(SONAME)
+else
+LDFLAGS_LIB += -Wl,-install_name,$(LIBDIR)/$(SONAME)
 endif
 
 test: test_g test_fast