Enable coverage when strace is self terminated by signal

The data collected by -fprofile-arcs during program run is saved
on program exit.  When program is terminated by signal, the data
is not saved.  However, libgcov provides an interface to dump
the data at runtime.

* m4/ax_code_coverage.m4 (AX_CODE_COVERAGE): Add -DENABLE_COVERAGE_GCOV
to CODE_COVERAGE_CPPFLAGS.
* strace.c [ENABLE_COVERAGE_GCOV] (__gcov_flush): New prototype.
(main) [ENABLE_COVERAGE_GCOV]: Call __gcov_flush() before raise()
and sigprocmask() calls that might cause program termination.

m4/ax_code_coverage.m4

@@ -140,7 +140,7 @@ AC_DEFUN([AX_CODE_COVERAGE],[
 		])
 
 		dnl Build the code coverage flags
-		CODE_COVERAGE_CPPFLAGS="-DNDEBUG"
+		CODE_COVERAGE_CPPFLAGS="-DENABLE_COVERAGE_GCOV -DNDEBUG"
 		CODE_COVERAGE_CFLAGS="-O0 -g -fprofile-arcs -ftest-coverage"
 		CODE_COVERAGE_CXXFLAGS="-O0 -g -fprofile-arcs -ftest-coverage"
 		CODE_COVERAGE_LDFLAGS="-lgcov"

strace.c

@@ -2512,6 +2512,10 @@ restart_tracee:
 	return true;
 }
 
+#ifdef ENABLE_COVERAGE_GCOV
+extern void __gcov_flush();
+#endif
+
 int
 main(int argc, char *argv[])
 {
@@ -2538,12 +2542,18 @@ main(int argc, char *argv[])
 		/* Child was killed by a signal, mimic that.  */
 		exit_code &= 0xff;
 		signal(exit_code, SIG_DFL);
+#ifdef ENABLE_COVERAGE_GCOV
+		__gcov_flush();
+#endif
 		raise(exit_code);
 
 		/* Unblock the signal.  */
 		sigset_t mask;
 		sigemptyset(&mask);
 		sigaddset(&mask, exit_code);
+#ifdef ENABLE_COVERAGE_GCOV
+		__gcov_flush();
+#endif
 		sigprocmask(SIG_UNBLOCK, &mask, NULL);
 
 		/* Paranoia - what if this signal is not fatal?