Resolve abi symbol references from miniruby to avoid circular deps

Adding `ruby` to `PREP` causes the following circular dependencies
because `PREP` is used as a prerequisite by some targets required to
build `ruby` target itself.
```
make: Circular .rbconfig.time <- ruby dependency dropped.
make: Circular builtin_binary.inc <- ruby dependency dropped.
make: Circular ext/extinit.c <- ruby dependency dropped.
make: Circular ruby <- ruby dependency dropped.
```

Adding a new Make variable like `EXTPREP` only for exts may be also
reasonable, but it would introduce another complexity into our build
system. `-bundle_loader` doesn't care that link-time and run-time
loader executables are different as long as bound symbols are provided,
so it's ok to resolve from miniruby to simplify our build.

configure.ac

@@ -3015,7 +3015,6 @@ STATIC=
                           # When building ext bundles, a mach-o bundle needs to know its loader
                           # program to bind symbols from the ruby executable
                           EXTDLDFLAGS="-bundle_loader '\$(BUILTRUBY)'"
-                          PREP="$PREP"' $(PROGRAM)'
 			])
 			rb_cv_dlopen=yes],
         [aix*], [	: ${LDSHARED='$(CC)'}
@@ -3340,7 +3339,7 @@ AS_IF([test x"$cross_compiling" = xyes], [
   AC_SUBST(XRUBY_LIBDIR)
   AC_SUBST(XRUBY_RUBYLIBDIR)
   AC_SUBST(XRUBY_RUBYHDRDIR)
-  PREP="$PREP "'$(arch)-fake.rb'
+  PREP='$(arch)-fake.rb'
   RUNRUBY_COMMAND='$(MINIRUBY) -I`cd $(srcdir)/lib; pwd`'
   RUNRUBY='$(RUNRUBY_COMMAND)'
   XRUBY='$(MINIRUBY)'
@@ -3350,7 +3349,7 @@ AS_IF([test x"$cross_compiling" = xyes], [
 ], [
   MINIRUBY='./miniruby$(EXEEXT) -I$(srcdir)/lib -I.'
   MINIRUBY="$MINIRUBY"' -I$(EXTOUT)/common'
-  PREP="$PREP "'miniruby$(EXEEXT)'
+  PREP='miniruby$(EXEEXT)'
   RUNRUBY_COMMAND='$(MINIRUBY) $(tooldir)/runruby.rb --extout=$(EXTOUT) $(RUNRUBYOPT)'
   RUNRUBY='$(RUNRUBY_COMMAND) --'
   XRUBY='$(RUNRUBY)'

enc/Makefile.in

@@ -36,7 +36,7 @@ RUBY_SO_NAME = @RUBY_SO_NAME@
 LIBRUBY = @LIBRUBY@
 LIBRUBYARG_SHARED = @LIBRUBYARG_SHARED@
 LIBRUBYARG_STATIC = $(LIBRUBYARG_SHARED)
-BUILTRUBY = $(topdir)/ruby$(EXEEXT)
+BUILTRUBY = $(topdir)/miniruby$(EXEEXT)
 
 empty =
 AR = @AR@

ext/extmk.rb

@@ -427,7 +427,7 @@ end
 topruby = $ruby
 $ruby = topruby.join(' ')
 $mflags << "ruby=#$ruby"
-$builtruby = '$(topdir)/ruby' + EXEEXT # Must be an executable path
+$builtruby = '$(topdir)/miniruby' + EXEEXT # Must be an executable path
 
 MTIMES = [__FILE__, 'rbconfig.rb', srcdir+'/lib/mkmf.rb'].collect {|f| File.mtime(f)}