[Bug #21092] Fallback variables after execonf has done

	When reading from a dummy makefile, the global variables initialized
	in `init_mkmf` may not be overridden.
	---
	 ext/extmk.rb | 6 +++---
	 1 file changed, 3 insertions(+), 3 deletions(-)

	Alias init functions

	The extension library has each initialization function named "Init_" +
	basename. If multiple extensions have the same base name (such as
	cgi/escape and erb/escape), the same function will be registered for
	both names.

	To fix this conflict, rename the initialization functions under sub
	directories using using parent names, when statically linking.

So different timestamps for different paths will be used.  Extentions
paths in bundled gems contain `ruby_version`, which includes the ABI
version, and the same timestamp file for different paths resulted in
build failures when it changed.

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.

For the macOS -bundle_loader linker option, we need a path to the
Ruby exectuable. $(RUBY) is not necessarily a path since it could
be a command line invocation. That happens during build with
runruby.rb and can happen post installation if the user passes
the --ruby option to a extconf.rb. Use $(bindir) to locate
the executable instead.

Before installation, $(bindir) doesn't exist, so we need to be
able to override $(BUILTRUBY) in such situations so test-spec
and bundled extensions could build. Use a new mkmf global,
$builtruby, to do this; set it in fake.rb and in extmk.rb.

Our build system is quite complex...