GNUmakefile: fix portability issues for out-of-tree and cross builds

[Ebola-Chan-bot]

Five small fixes to make GNUmakefile work correctly in out-of-tree builds (make -f /path/to/src/GNUmakefile) and cross-compilation setups where the toolchain has a $(CROSS_COMPILE) prefix.

1. Fix source path duplication in COMPILE macro

# Before
COMPILE = ... -c $(SRC)$< -o $@
# After
COMPILE = ... -c $< -o $@

VPATH already causes make to resolve $< to the full source path. Prepending $(SRC) duplicates the prefix in out-of-tree builds, producing paths like ../../src/../../src/cli/cli.c.

2. Use $(VPATH) instead of $(SRC) in define_from_arch.h

# Before
$2$1 := $(shell $(CC) $1 -E -dM -DNO_LIBC_HEADER $(SRC)/arch.h | ...)
# After
$2$1 := $(shell $(CC) $1 -E -dM -DNO_LIBC_HEADER $(VPATH)/arch.h | ...)

The comment on line 4 says "the VPATH variable must point to the actual makefile directory", making $(VPATH) the designated source-directory variable. CPPFLAGS already uses -I$(VPATH). This change aligns define_from_arch.h with the rest of the file.

3. Replace deprecated egrep with grep -E

# Before
USE_BUILD_H := $(patsubst ...,$(shell egrep -sl ...))
# After
USE_BUILD_H := $(patsubst ...,$(shell grep -E -sl ...))

egrep has been deprecated by POSIX (IEEE Std 1003.1-2024) and GNU grep now emits a warning. On some systems (e.g. MSYS2/Git-for-Windows), egrep is a wrapper shell script whose shebang path may contain spaces, which breaks $(shell) invocations on native-Windows make implementations.

4. Add READELF variable

READELF  ?= $(CROSS_COMPILE)readelf

This follows the existing pattern for CC, STRIP, OBJCOPY, and OBJDUMP, which all use ?= with $(CROSS_COMPILE) prefix. The hardcoded readelf in the loader-info.c rule is replaced with $(READELF).

5. Use $(VPATH) for loader-info.awk path

# Before
readelf -s $< | awk -f loader/loader-info.awk > $@
# After
$(READELF) -s $< | awk -f $(VPATH)/loader/loader-info.awk > $@

In out-of-tree builds, the awk script is in the source tree, not the build directory. Without $(VPATH), awk -f loader/loader-info.awk fails with "file not found".

[Copilot]

Pull request overview

Improves src/GNUmakefile portability and correctness for out-of-tree builds and cross-compilation by relying on VPATH-resolved paths and toolchain-prefixed utilities.

Changes:

• Use $(VPATH) (not $(SRC)) for source-directory references where the Makefile declares VPATH as the canonical makefile/source dir.
• Fix out-of-tree compile path handling by compiling $< directly (letting VPATH resolve the correct source path).
• Replace deprecated egrep usage with grep -E, and add a configurable READELF for cross builds (also VPATH-prefix the AWK script path).

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