[Bug 255913] devel/aws-c-common: let CMake detect -moutline-atomics correctly

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=255913

            Bug ID: 255913
           Summary: devel/aws-c-common: let CMake detect -moutline-atomics
                    correctly
           Product: Ports & Packages
           Version: Latest
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: Individual Port(s)
          Assignee: danilo at FreeBSD.org
          Reporter: dim at FreeBSD.org
          Assignee: danilo at FreeBSD.org
             Flags: maintainer-feedback?(danilo at FreeBSD.org)

Created attachment 224980
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=224980&action=edit
Let CMake detect -moutline-atomics correctly

During an exp-run for llvm 12 (see bug 255570), it turned out that
security/aws-c-cal does not build with clang 12.0.0 [1], at least not on amd64:

-- The C compiler identification is Clang 12.0.0
[...]
-- Performing Test HAS_MOUTLINE_ATOMICS
-- Performing Test HAS_MOUTLINE_ATOMICS - Success
[...]
FAILED: CMakeFiles/aws-c-cal.dir/source/der.c.o 
/usr/bin/cc -DAWS_CAL_EXPORTS -DAWS_CAL_USE_IMPORT_EXPORT
-DAWS_COMMON_USE_IMPORT_EXPORT -DHAVE_SYSCONF -Daws_c_cal_EXPORTS
-I/wrkdirs/usr/ports/security/aws-c-cal/work/aws-c-cal-0.4.5/include -isystem
/usr/local/include -O2 -pipe  -fstack-protector-strong -fno-strict-aliasing -O2
-pipe  -fstack-protector-strong -fno-strict-aliasing -fPIC -Wall -Werror
-Wstrict-prototypes -fno-omit-frame-pointer -pedantic -Wno-long-long -fPIC
-moutline-atomics -Wgnu -Wno-gnu-zero-variadic-macro-arguments
-fvisibility=hidden -std=gnu99 -MD -MT CMakeFiles/aws-c-cal.dir/source/der.c.o
-MF CMakeFiles/aws-c-cal.dir/source/der.c.o.d -o
CMakeFiles/aws-c-cal.dir/source/der.c.o -c
/wrkdirs/usr/ports/security/aws-c-cal/work/aws-c-cal-0.4.5/source/der.c
cc: error: The 'x86_64' architecture does not support -moutline-atomics; flag
ignored [-Werror,-Woption-ignored]

This port uses /usr/local/lib/cmake/AwsCFlags.cmake to detect the
-moutline-atomics flag, but it does so by checking if the compiler prints an
*error* when using the flag. It could be debated that CMake's
check_c_compiler_flag() functionality is rather broken in that sense, as it
does not exhaustively scan for warnings from the compiler.

On the other hand, security/aws-c-cal compiles with -Werror, so then you get an
error for such warnings. Instead of hacking around it in security/aws-c-cal, I
propose to fix it in devel/aws-c-common instead, simply by adding -Werror to
the HAS_MOUTLINE_ATOMICS check.

This will ensure that it does *not* get enabled on x86, where it is not
functional, and will still get enabled on e.g. aarch64, for which this flag is
meant.

[1]
http://package22.nyi.freebsd.org/data/mainamd64PR255570-default/2021-05-08_16h02m24s/logs/errors/aws-c-cal-0.4.5.log

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