arsig: fix macOS (Apple clang) build of xgb_model.c

[ProfessionalSeaweedDevourer]

Problem

On macOS, the C compiler invoked as gcc is Apple clang. Building from a
clean tree fails in arsig while compiling the generated src/arsig/xgb_model.c
(the SVM/XGBoost integer-ambiguity-validation model):

xgb_model.c:102084:298: fatal error: bracket nesting level exceeded maximum of 256
xgb_model.c:102084:298: note: use -fbracket-depth=N to increase maximum nesting level
1 error generated.

Apple clang caps C bracket nesting at 256; that generated file exceeds it.
GNU gcc (Linux) has no such default limit, which is why CI/Linux builds are
unaffected.

Why the obvious one-liner is not enough

Adding -fbracket-depth=1024 lifts the 256 limit, but clang parses via
recursive descent, and on the default macOS stack of 8 MB (ulimit -s
= 8192) the parser then overflows the stack and segfaults on this very
deeply nested file:

clang: error: unable to execute command: Segmentation fault: 11
clang: error: clang frontend command failed due to signal

So a robust fix must (a) raise the bracket-depth limit and (b) give the
compiler enough stack.

Fix (src/arsig/Makefile, no effect on GNU/Linux)

• add -fbracket-depth=1024 only when the C compiler is clang (GNU gcc has
  no such limit and rejects the flag with unrecognized command-line option);
• raise the stack soft limit to the hard limit for the C compile step so
  clang's recursive parser does not overflow.

CFLAGS = -std=c99
ifneq (,$(findstring clang,$(shell gcc --version 2>/dev/null)))
  CFLAGS += -fbracket-depth=1024
endif
...
$(OBJ_PATH)%.o : %.c
	$(DIR_GUARD)
	ulimit -s $$(ulimit -Hs) 2>/dev/null || true; gcc $(CFLAGS) -c $< -o $@

Testing (macOS arm64, Apple clang 17; Homebrew GNU gcc-16)

compiler	flag	stack	result
Apple clang	none (current)	8 MB	fail: bracket nesting > 256
GNU gcc-16	none	8 MB	OK (no limit; flag not needed)
GNU gcc-16	-fbracket-depth	8 MB	fail: unrecognized option (flag must be clang-only)
Apple clang	-fbracket-depth	8 MB	segfault (parser stack overflow)
Apple clang	-fbracket-depth	hard (64 MB)	OK

With this patch, a full install.sh build completes on macOS under the default
8 MB stack, and GNU gcc builds the file unchanged (flag not added).

Alternative

The stack bump could instead live once in install.sh (global ulimit -s
before the build) rather than in the arsig recipe; happy to move it there if
preferred. Opened as a draft for maintainer feedback on the approach.