Fix sanitizer builds with clang

[zuiderkwast]

By including <stdatomic.h> after the other includes in the unit test, we can avoid redefining a macro which led to a build failure.

Fixes #1394

[zuiderkwast]

We have #include <stdatomic.h> in several files now, so I wonder why we still have this conditional code in the Makefile:

# Detect if the compiler supports C11 _Atomic.
# NUMBER_SIGN_CHAR is a workaround to support both GNU Make 4.3 and older versions.
NUMBER_SIGN_CHAR := \#
C11_ATOMIC := $(shell sh -c 'echo "$(NUMBER_SIGN_CHAR)include <stdatomic.h>" > foo.c; \
	$(CC) -std=gnu11 -c foo.c -o foo.o > /dev/null 2>&1; \
	if [ -f foo.o ]; then echo "yes"; rm foo.o; fi; rm foo.c')
ifeq ($(C11_ATOMIC),yes)
	STD+=-std=gnu11
else
	STD+=-std=c99
endif

If we can't compile the foo.c test file here, we can't compile replication.c and other files, so we're effectively requiring it and we're always building with -std=gnu11 now?

[zuiderkwast]

@uriyage FYI this fixes it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.82%. Comparing base (6df376d) to head (17b07d0).
Report is 13 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1402      +/-   ##
============================================
- Coverage     70.82%   70.82%   -0.01%     
============================================
  Files           118      118              
  Lines         63561    63561              
============================================
- Hits          45020    45016       -4     
- Misses        18541    18545       +4     

see 13 files with indirect coverage changes