meson: explicitly include coverage tweaks when built w/ --coverage

To make sure we don't miss any _exit() calls let's move the
coverage-related tweaks into a separate header file and include it
explicitly on the compiler command line using -include when a coverage
build is requested.

Follow-up to c6552ad381.

src/basic/coverage.h

@@ -0,0 +1,19 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+/* When built with --coverage (gcov) we need to explicitly call __gcov_dump()
+ * in places where we use _exit(), since _exit() skips at-exit hooks resulting
+ * in lost coverage.
+ *
+ * To make sure we don't miss any _exit() calls, this header file is included
+ * explicitly on the compiler command line via the -include directive (only
+ * when built with -Db_coverage=true)
+ * */
+extern void _exit(int);
+extern void __gcov_dump(void);
+
+static inline void _coverage__exit(int status) {
+        __gcov_dump();
+        _exit(status);
+}
+#define _exit(x) _coverage__exit(x)