Add GCC example with scoped configs and assembly support

[typeless]

Summary

• Add examples/gcc/ demonstrating a real-world multi-directory build: GMP + MPFR + MPC from GCC's bundled sources
• Extend putup configure to copy subdir tup.config files for scoped config merging

Build Features Demonstrated

• Assembly support: CPU-specific .asm → m4 → assembler pipeline with config-driven source selection
• foo-y conditional compilation: Kbuild-inspired pattern for toggling multi-function sources without ifeq
• Multi-variant parallel builds: Multiple -B directories with different toolchain configs
• Per-component scoped configs: Each library has its own tup.config, merged with root via scoped config merging
• Self-contained libraries: Each library buildable standalone or composed under root
• 3-tree builds: Tupfiles (-C), GCC sources (-S), and build output (-B) in separate directories
• Cross-directory groups: Subdirectory objects collected via ../<objs> into parent archive

Files

Area	Files	Purpose
Build driver	Makefile.pup, scripts/resolve-mpn.sh	Make wrapper + mpn source resolution
Root config	Tuprules.tup, Tupfile, configs/*.config	Toolchain vars, library directory layout
GMP	gmp/{Tupfile,Tuprules.tup,tup.config}, gmp/{mpn,mpz,mpq,mpf,printf,scanf,rand}/Tupfile	8-directory build with assembly support
MPFR	mpfr/{Tupfile,Tuprules.tup,tup.config,src/Tupfile}	Depends on GMP headers
MPC	mpc/{Tupfile,Tuprules.tup,tup.config,src/Tupfile}	Depends on GMP + MPFR headers
Configure	src/cli/cmd_configure.cpp	Copy subdir tup.config files to build tree

Test plan

• make -f Makefile.pup SRCDIR=../gcc-15.2.0 builds all three libraries in generic C mode
• make -f Makefile.pup MPN_CPU=x86_64 SRCDIR=../gcc-15.2.0 builds with x86-64 assembly
• make -f Makefile.pup multi SRCDIR=../gcc-15.2.0 builds x86_64-linux + aarch64-linux variants
• Standalone cd gmp && putup configure && putup works with ?= defaults
• make test passes (no regressions in existing tests)

🤖 Generated with Claude Code

[typeless]

GCC Example Build — Handoff Note

Current State

Build reaches ~1088/1740 commands before failing. The build command:

cd /home/mural/src/pup/examples/gcc && \
  PUP_IMPLICIT_DEPS=0 /home/mural/src/pup/build/putup \
  -C . -S /home/mural/src/gcc-15.2.0 -B ../../build-gcc -j$(nproc)

You need a GCC 15.2.0 source tree at /home/mural/src/gcc-15.2.0 (adjust -S path as needed).

Next Issue to Fix

4 generators with file-output flags write relative to CWD (read-only in 3-tree mode)

These generators use flags like -O, -H, -A, -D, -L, -h, -c to write output files relative to their working directory. In 3-tree mode, CWD = source dir (read-only).

Affected rules in examples/gcc/gcc/Tupfile (lines ~408-459):

Generator	Flags	Outputs
genattrtab	-A, -D, -L	insn-attrtab.cc, insn-dfatab.cc, insn-latencytab.cc
genopinit	-h, -c	insn-opinit.h, insn-opinit.cc
genemit	-O (x10)	insn-emit-{1..10}.cc
genrecog	-H, -O (x10)	insn-recog.h, insn-recog-{1..10}.cc

Fix: Use the same SRCDIR=$PWD && cd $(TUP_VARIANT_OUTPUTDIR) pattern already applied to genmatch (lines 308-332). Check whether %f gives absolute or relative paths in 3-tree mode — if $(MD) already resolves to absolute source paths, a simple cd $(TUP_VARIANT_OUTPUTDIR) may suffice. Test with a single generator first.

Key Discoveries (This Build Effort)

1. $VAR (bare dollar) passes through to shell — putup's expand() only expands $(VAR). Bare $PWD, $SRCDIR etc. pass through as shell variables.

2. Display text ^ ... ^ must be on the SAME line as |> — if on a continuation line, it gets included in the shell command and causes /bin/sh: ^: not found.

3. GCC generators need BOTH -DIN_GCC AND -DGENERATOR_FILE — already set in HOST_CXXFLAGS in gcc/Tuprules.tup.

4. tm_p.h must include 3 headers: config/i386/i386-protos.h, config/linux-protos.h, and tm-preds.h (declares predicate functions like register_operand).

5. Don't set CONFIG_HAVE_UCHAR=1 — modern Linux doesn't provide uchar. GCC's coretypes.h typedefs it when HAVE_UCHAR is NOT defined.

6. After editing tup.config, must re-run putup configure to propagate to build dir.

Workflow

Build → read first error → diagnose → fix Tupfile/config → rebuild. Repeat until cc1 links.

Separate Issue: 3-Tree Groups Bug

Order-only groups produce "has no members" warnings in 3-tree mode. Plan exists at .claude/plans/snuggly-noodling-scroll.md. This is a putup core bug, separate from the GCC example work. The GCC example uses PUP_IMPLICIT_DEPS=0 as a workaround.