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

src/bash.h

@@ -0,0 +1,31 @@
+#ifndef __BASH_H__
+#define __BASH_H__
+
+#define EXECUTION_SUCCESS 0
+#define EXECUTION_FAILURE 1
+#define EX_USAGE 258
+
+#define BUILTIN_ENABLED 1
+
+typedef struct word_desc {
+  char *word;
+  int flags;
+} WORD_DESC;
+
+typedef struct word_list {
+  struct word_list *next;
+  WORD_DESC *word;
+} WORD_LIST;
+
+typedef int sh_builtin_func_t(WORD_LIST *);
+
+struct builtin {
+  char *name;
+  sh_builtin_func_t *function;
+  int flags;
+  char * const *long_doc;
+  const char *short_doc;
+  char *unused;
+};
+
+#endif

src/realpath.c

@@ -0,0 +1,43 @@
+#include "bash.h"
+#include <stdlib.h>
+#include <stdio.h>
+
+int realpath_builtin(list)
+WORD_LIST *list;
+{
+	int es;
+	char *realbuf, *p;
+
+	if (list == 0) {
+		// builtin_usage();
+		return (EX_USAGE);
+	}
+
+	for (es = EXECUTION_SUCCESS; list; list = list->next) {
+		p = list->word->word;
+		realbuf = realpath(p, NULL);
+		if (realbuf == NULL) {
+			es = EXECUTION_FAILURE;
+			// builtin_error("%s: cannot resolve: %s", p, strerror(errno));
+		} else {
+			printf("%s\n", realbuf);
+			free(realbuf);
+		}
+	}
+	return es;
+}
+
+char *realpath_doc[] = {
+	"Display each PATHNAME argument, resolving symbolic links. The exit status",
+	"is 0 if each PATHNAME was resolved; non-zero otherwise.",
+	(char *)NULL
+};
+
+struct builtin realpath_struct = {
+	"realpath",
+	realpath_builtin,
+	BUILTIN_ENABLED,
+	realpath_doc,
+	"realpath pathname [pathname...]",
+	0
+};