[PATCH] Makefile: make COMPUTE_HEADER_DEPENDENCIES=auto work with DEVOPTS=pedantic

The "COMPUTE_HEADER_DEPENDENCIES" feature added in [1] was extended to
use auto-detection in [2]. Then when -Wpedantic support was added to
DEVOPTS in [3] we started passing -Wpedantic in combination with
-Werror to the compiler here.

This broke the auto-detection, but since we'd quieted it in [4] we
didn't find out. It was emitting all of this on STDERR under GCC:

    /dev/null:1: error: ISO C forbids an empty translation unit
    [-Werror=pedantic]
    cc1: note: unrecognized command-line option
    ‘-Wno-pedantic-ms-format’ may have been intended to silence
    earlier diagnostics
    cc1: all warnings being treated as errors

Let's fix that bug by maintaining a NON_DEVELOPER_CFLAGS, it's like
ALL_CFLAGS but without anything we add in config.mak.dev, and
furthermore stop redirecting STDERR to /dev/null, this means that
someone whose compiler doesn't support this will see this output, but
also this new message:

    Non-zero 1 exit with COMPUTE_HEADER_DEPENDENCIES=auto, set it to "yes" or "no" to quiet auto-detect

It's also possible that some compilers will emit warnings but still
give a zero exit code, anyone using a compiler like that will
potentially get more verbose output from the Makefile until they set
COMPUTE_HEADER_DEPENDENCIES=no. E.g. on AIX's xlc we'll now emit:

    /opt/IBM/xlc/13.1.3/bin/.orig/xlc: 1501-208 (S) command option D is missing a subargument
    Non-zero 40 exit with COMPUTE_HEADER_DEPENDENCIES=auto, set it to "yes" or "no" to quiet auto-detect

And on Solaris with SunCC:

    cc: Warning: Option -x passed to ld, if ld is invoked, ignored otherwise
    cc: refused to overwrite input file by output file: /dev/null
    cc: Warning: Option -x passed to ld, if ld is invoked, ignored otherwise
    cc: refused to overwrite input file by output file: /dev/null
    Non-zero 1 exit with COMPUTE_HEADER_DEPENDENCIES=auto, set it to "yes" or "no" to quiet auto-detect

Both are quieted by setting COMPUTE_HEADER_DEPENDENCIES=no as
suggested.

I considered piping the output and the exit code to a variable
instead, but e.g. under GCC that would lose the coloring in the error
messages.

1. f2fabbf76e4 (Teach Makefile to check header dependencies,
   2010-01-26)
2. 111ee18c31f (Makefile: Use computed header dependencies if the
   compiler supports it, 2011-08-18)
3. 729b3925ed9 (Makefile: add a DEVOPTS flag to get pedantic
   compilation, 2018-07-24)
4. 6a8cbc41bac (developer: enable pedantic by default, 2021-09-03)

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
---
 Makefile | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/Makefile b/Makefile
index 9df565f27bb..c30f3b8b204 100644
--- a/Makefile
+++ b/Makefile

@@ -1242,7 +1242,8 @@ ALL_COMMANDS_TO_INSTALL += git-upload-archive$(X)
 ALL_COMMANDS_TO_INSTALL += git-upload-pack$(X)
 endif
 
-ALL_CFLAGS = $(DEVELOPER_CFLAGS) $(CPPFLAGS) $(CFLAGS)
+NON_DEVELOPER_CFLAGS = $(CPPFLAGS) $(CFLAGS)
+ALL_CFLAGS = $(DEVELOPER_CFLAGS) $(NON_DEVELOPER_CFLAGS)
 ALL_LDFLAGS = $(LDFLAGS)
 
 comma := ,

@@ -1277,13 +1278,14 @@ COMPUTE_HEADER_DEPENDENCIES = auto
 endif
 
 ifeq ($(COMPUTE_HEADER_DEPENDENCIES),auto)
-dep_check = $(shell $(CC) $(ALL_CFLAGS) \
+dep_check = $(shell $(CC) $(NON_DEVELOPER_CFLAGS) \
 	-c -MF /dev/null -MQ /dev/null -MMD -MP \
-	-x c /dev/null -o /dev/null 2>&1; \
+	-x c /dev/null -o /dev/null; \
 	echo $$?)
 ifeq ($(dep_check),0)
 override COMPUTE_HEADER_DEPENDENCIES = yes
 else
+$(info Non-zero $(dep_check) exit with COMPUTE_HEADER_DEPENDENCIES=auto, set it to "yes" or "no" to quiet auto-detect)
 override COMPUTE_HEADER_DEPENDENCIES = no
 endif
 endif

-- 
2.33.0.1225.g17f21f53d74