Speed up realpath() with dynamically loaded C extension

On systems that support both C compiling and dynamic loading, we can
speed up `realpath()` (where most time in rbenv is spent) by replacing
it with a dynamically loaded bash builtin.

When `make -C src` is called in the project's root,
`libexec/rbenv-realpath.dylib` will be created. If it exists, rbenv will
attempt to load it as a builtin command. If it fails, execution will
fall back to the old `realpath()` shell function.

src/Makefile

@@ -0,0 +1,10 @@
+SHOBJ_LDFLAGS = -dynamiclib -current_version 1.0
+
+.c.o:
+	$(CC) $(CFLAGS) -c -o $@ $<
+
+../libexec/rbenv-realpath.dylib: realpath.o
+	$(CC) $(CFLAGS) $(SHOBJ_LDFLAGS) -o $@ realpath.o
+
+clean:
+	rm -f *.o ../libexec/*.dylib