This commit introduces two user visible changes.

1. Split the stem for static pattern rules to dirname and basename.
With this feature static pattern rules do the same stem splitting and directory
prefix reconstruction as implicit search does for pattern rules.
This feature allows the same static pattern rule be used for targets in
different directories and with different prefixes.

$ ls
makefile
$ cat makefile
.SPLIT_STEM_FOR_STATIC_PATTERN_RULES:
obj:=drivers/hello.x pilots/video/world.x
all:$(obj)
$(obj): %.x: pre-%.q .%.d; $(info $@ from $^)
drivers/pre-hello.q drivers/.hello.d pilots/video/pre-world.q pilots/video/.world.d:;
$ make
drivers/hello.x from drivers/pre-hello.q drivers/.hello.d
pilots/video/world.x from pilots/video/pre-world.q pilots/video/.world.d
make: Nothing to be done for 'all'.

Suggested by Masahiro Yamada <masahiroy@kernel.org>.
https://lists.gnu.org/archive/html/help-make/2024-06/msg00018.html.


2. Allow a backslash to escape % in a prerequisite pattern of a static pattern
rule when second expansion is enabled. This allows static pattern rules to have
the same behavior whether second expansion is enabled or not.

$ ls
makefile
$ cat makefile

.SECONDEXPANSION:
all: hello.x
hello.x: %.x: $$(strip \%-%.q); $(info $@ from $<)
%-hello.q:;
$ make
hello.x from %-hello.q
make: Nothing to be done for 'all'.