Suppress gcc 15 unterminated-string-initialization warnings

	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

By replacing `ALLOBJS` suffix with intermediate file suffixes instead
of roughly removing by wildcards.  Made `cleanlibs` append `.dSYM`
suffix for each word in `TARGET_SO`, not the end of the entire list.

Split `PACKED_STRUCT` and `PACKED_STRUCT_UNALIGNED` macros into the
macros bellow:
* `RBIMPL_ATTR_PACKED_STRUCT_BEGIN`
* `RBIMPL_ATTR_PACKED_STRUCT_END`
* `RBIMPL_ATTR_PACKED_STRUCT_UNALIGNED_BEGIN`
* `RBIMPL_ATTR_PACKED_STRUCT_UNALIGNED_END`

This reverts commit 32ad8df9d1e07e1b2435a8890d070802fcd2989f,
which broke out-of-src build with the pre-generated transcoder sources.

When out-of-src build, at the beginning of a build, `make -f enc.mk
srcs` generates trans C sources under build dir.

On the other hand, enc/trans/*.o were built from trans C sources
generated under srcdir due to the following auto-generated rules from
enc/depend.

```
encsrcdir = ../src/enc
...
enc/trans/big5.$(OBJEXT): $(encsrcdir)/trans/big5.c
```

Therefore, trans C sources are generated twice under srcdir and build
dir during a build.

Ideally, trans C sources have always been built before compilation of
enc/trans/*.o because the source generation is prereq, so making
enc/trans/*.o doesn't trigger trans C source generation and shouldn't
require MINIRUBY as a make arg for enc.mk. However, the second trans C
source gen is unintentionally triggered by enc/trans/*.o, so `make -f
enc.mk libencs` requires MINIRUBY for now.

When no `--with-static-linked-ext`, `make -f enc.mk libencs` is
triggered from common.mk with MINIRUBY, so there is no problem.
But when `--with-static-linked-ext`, libencs should be statically-linked
to ruby, so `make -f enc.mk libencs` is triggered from exts.mk, and
exts.mk invokes it without MINIRUBY.

Therefore, when out-of-src build and with `--with-static-linked-ext`,
the second trans C source gen fails due to missing MINIRUBY.
This issue is deterministically reproducible without -j because
common.mk's `main` rule also has libencs prerequisite.

This patch supresses the second trans C source gen.