coverage: atomically update gcov files

problem: we keep getting merge issues with gcov files.  It seems the
default for updating these is "single" meaning single threaded unless
the compilation is done with `-pthread`. I'm guessing we see the errors
most on libraries we use in multiple places that are not themselves
compiled that way, even though they're eventually linked into binaries
that are. If this doesn't cover it, we should also do what's listed in

solution: add `-fprofile-update=atomic` to our coverage flags

Author: trws

config/ax_code_coverage.m4

@@ -125,10 +125,10 @@ AC_DEFUN([AX_CODE_COVERAGE],[
 		dnl Build the code coverage flags
 		dnl Define CODE_COVERAGE_LDFLAGS for backwards compatibility
 		CODE_COVERAGE_CPPFLAGS="-DNDEBUG"
-		CODE_COVERAGE_CFLAGS="-O0 -g -fprofile-arcs -ftest-coverage"
-		CODE_COVERAGE_CXXFLAGS="-O0 -g -fprofile-arcs -ftest-coverage"
+		CODE_COVERAGE_CFLAGS="-O0 -g -fprofile-arcs -ftest-coverage -fprofile-update=atomic"
+		CODE_COVERAGE_CXXFLAGS="-O0 -g -fprofile-arcs -ftest-coverage -fprofile-update=atomic"
 		CODE_COVERAGE_LIBS="-lgcov"
-		CODE_COVERAGE_LDFLAGS="$CODE_COVERAGE_LIBS"
+		CODE_COVERAGE_LDFLAGS="$CODE_COVERAGE_LIBS -fprofile-update=atomic"
 
 		AC_SUBST([CODE_COVERAGE_CPPFLAGS])
 		AC_SUBST([CODE_COVERAGE_CFLAGS])