[Bug #21779] Uniquify `InitVM` functions as well as `Init`

	Avoid possible name conflict when `--with-static-linked-ext`.

	[Bug #21779] Do not export InitVM functions

	Fix ruby/io-console#105.

The combination of mingw tools and cygin/msys2 ruby leaves CRs.

	[Bug #21546] Make the generated pc file relocatable

	Suppress gcc 15 unterminated-string-initialization warnings

	Drop an ignored attribute
	MIME-Version: 1.0
	Content-Type: text/plain; charset=UTF-8
	Content-Transfer-Encoding: 8bit

	GCC 13.3.0 (Ubuntu 24.04) emits the following warning:

	../symbol.c: In function ‘rb_id_attrset’:
	../symbol.c:175:9: warning: ‘nonstring’ attribute ignored on objects of type ‘const char[][8]’ [-Wattributes]
	  175 |         RBIMPL_ATTR_NONSTRING() static const char id_types[][8] = {
	      |         ^~~~~~~~~~~~~~~~~~~~~

	Separate `__has_attribute` from `defined(__has_attribute)`

	Fix Visual C warnings:
	```
	regenc.h(121): warning C4067: unexpected tokens following preprocessor directive - expected a newline
	```

	Cast up `int` instruction code to `VALUE`

	Fix Visual C warnings:
	```
	iseq.c(3793): warning C4312: 'type cast': conversion from 'int' to 'void *' of greater size
	iseq.c(3794): warning C4312: 'type cast': conversion from 'int' to 'void *' of greater size
	```

	Do not let files depend on a phony target

	Detect `clock_gettime` and `clock_getres` for winpthreads

	digest.so needs ruby/digest.h which is installed by build-ext

	Copy to path with the base name

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.

This reverts commit 5bb946228550c7f171c27725860b153a675404f3.

Fixes https://bugs.ruby-lang.org/issues/20072

This reverts commit 1d5598fe0d3470e7cab06a756d40a9221fcd501b.