[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 <avarab@gmail.com>
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 Makefile | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)