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.

Introduce own global variable that contains the program name. If
it is unset then use `getprogname()` or `program_invocation_name`.

Co-authored-by: Ken Sedgwick <ken@bonsai.com>

Boss/Main.cpp

@@ -20,6 +20,8 @@
 # include "config.h"
 #endif
 
+std::string g_argv0;
+
 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

@@ -6,8 +6,15 @@
 #include <iomanip>
 #include <memory>
 #include <sstream>
-#include <vector>
+#include <stdlib.h>
 #include <string.h>
+#include <string>
+
+#ifdef HAVE_CONFIG_H
+# include"config.h"
+#endif
+
+extern std::string g_argv0;
 
 namespace Util {
 
@@ -74,13 +81,29 @@ class BacktraceException : public T {
 		return oss.str();
 	}
 
+        std::string progname() const {
+		// This variable is set by the CLBOSS mainline.
+		if (!g_argv0.empty()) {
+			return g_argv0;
+		}
+
+		// When libclboss is linked with the unit tests the
+		// g_argv0 variable is not set and we need to use
+		// built-in versions
+#ifdef HAVE_GETPROGNAME
+		return getprogname();
+#else
+		return program_invocation_name;
+#endif
+	}
+
 	// Unfortunately there is no simple way to get a high quality
 	// backtrace using in-process libraries.  Instead for now we
 	// popen an addr2line process and use it's output.
 	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", progname().c_str(), addr);
 
 		std::array<char, 128> buffer;
 		std::string result;

configure.ac

@@ -100,6 +100,21 @@ AC_LINK_IFELSE([AC_LANG_PROGRAM([[
 	AC_MSG_RESULT([no])
 ])
 
+# Check for getprogname()
+AC_MSG_CHECKING([for getprogname])
+AC_LINK_IFELSE([AC_LANG_PROGRAM([[
+#include<stdlib.h>
+]], [[
+	const char* p = getprogname();
+	(void)p; // avoid warning about set but unused variable
+]])], [ #then
+	AC_DEFINE([HAVE_GETPROGNAME], [1],
+		  [Define to 1 if you have a getprogname function.])
+	AC_MSG_RESULT([yes])
+], [ #else
+	AC_MSG_RESULT([no])
+])
+
 AC_CONFIG_FILES([Makefile
 		 external/bitcoin-ripemd160/Makefile
 		 external/bitcoin-sha256/Makefile