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], that "auto" detection has always piped
STDERR to /dev/null, so any failures on compilers that didn't support
these GCC flags would silently fall back to
"COMPUTE_HEADER_DEPENDENCIES=no".

Later when -Wpedantic support was added to DEVOPTS in [3] we started
passing -Wpedantic in combination with -Werror to the compiler
here. Note (to the pedantic): [3] actually passed "-pedantic", but it
and "-Wpedantic" are synonyms.

Turning on -Wpedantic in [3] broke the auto-detection, since this
relies on compiling an empty program. GCC would loudly complain on
STDERR:

    /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

But as that ended up in the "$(dep_check)" variable due to the "2>&1"
in [2] we didn't see it.

Then when [4] made DEVOPTS=pedantic the default specifying
"DEVELOPER=1" would effectively set "COMPUTE_HEADER_DEPENDENCIES=no".

To fix these issues let's unconditionally pass -Wno-pedantic after
$(ALL_CFLAGS), we might get a -Wpedantic via config.mak.dev after, or
the builder might specify it via CFLAGS. In either case this will undo
current and future problems with -Wpedantic.

I think it would make sense to simply remove the "2>&1", it would mean
that anyone using a non-GCC-like compiler would get warnings under
COMPUTE_HEADER_DEPENDENCIES=auto, e.g on AIX's xlc would 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 could be quieted by setting COMPUTE_HEADER_DEPENDENCIES=no
explicitly, as suggested, but let's see if this'll fix it without
emitting too much noise at those that aren't using "gcc" or "clang".

1. f2fabbf76e (Teach Makefile to check header dependencies,
   2010-01-26)
2. 111ee18c31 (Makefile: Use computed header dependencies if the
   compiler supports it, 2011-08-18)
3. 729b3925ed (Makefile: add a DEVOPTS flag to get pedantic
   compilation, 2018-07-24)
4. 6a8cbc41ba (developer: enable pedantic by default, 2021-09-03)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1278,6 +1278,7 @@ endif
 
 ifeq ($(COMPUTE_HEADER_DEPENDENCIES),auto)
 dep_check = $(shell $(CC) $(ALL_CFLAGS) \
+	-Wno-pedantic \
 	-c -MF /dev/null -MQ /dev/null -MMD -MP \
 	-x c /dev/null -o /dev/null 2>&1; \
 	echo $$?)