fix {depth} display, warnings and doc

- fix call stack depth display with {depth}: remove (5) stripped depths.
- fix warnings in tests
- Explain CLUTCHLOG_HAVE_UNIX_SYSINFO in the README.

tests/t-assert.cpp

@@ -0,0 +1,42 @@
+#include <iostream>
+#include <cassert>
+
+#include "../clutchlog/clutchlog.h"
+
+// Make asserts (de)clutchable.
+#define ASSERT(LEVEL, ...) { CLUTCHFUNC(LEVEL, assert, __VA_ARGS__) }
+
+void h()
+{
+    CLUTCHLOG(info, "!");
+    ASSERT(info, true == true);
+    std::clog << "--" << std::endl;
+}
+
+void g()
+{
+    CLUTCHLOG(warning, "world");
+    ASSERT(warning, strcmp("life","life") == 0);
+    h();
+}
+
+void f()
+{
+    CLUTCHLOG(error, "hello ");
+    ASSERT(error, strcmp("no more","please")!=0);
+    g();
+}
+
+int main(/*const int argc, char* argv[]*/)
+{
+    auto& log = clutchlog::logger();
+
+    log.func("f");
+    f();
+
+    log.func("g");
+    f();
+
+    log.func("h");
+    f();
+}