Avoid program_invocation_name which is available only on Linux

`program_invocation_name` is defined by the system headers only on
Linux, or at least it is not available on FreeBSD.

Thus, maintain own global variable that contains the program name.

Boss/Main.cpp

@@ -20,6 +20,8 @@
 # include "config.h"
 #endif
 
+std::string g_argv0{"unknown"};
+
 namespace Boss {
 
 class Main::Impl {
@@ -62,6 +64,7 @@ class Main::Impl {
 	      {
 		assert(argv.size() >= 1);
 		argv0 = argv[0];
+		g_argv0 = argv[0];
 		if (argv.size() >= 2) {
 			auto argv1 = argv[1];
 			if (argv1 == "--version" || argv1 == "-V")

Util/BacktraceException.hpp

@@ -8,6 +8,9 @@
 #include <sstream>
 #include <vector>
 #include <string.h>
+#include <string>
+
+extern std::string g_argv0;
 
 namespace Util {
 
@@ -80,7 +83,7 @@ class BacktraceException : public T {
 	std::string addr2line(void* addr) const {
 		char cmd[512];
 		snprintf(cmd, sizeof(cmd),
-			 "addr2line -C -f -p -e %s %p", program_invocation_name, addr);
+			 "addr2line -C -f -p -e %s %p", g_argv0.c_str(), addr);
 
 		std::array<char, 128> buffer;
 		std::string result;