Fixed build with MinGW toolchain

As shikadilord pointed out, the std::exception constructors from
strings are a Microsoft extension. I replaced std::exception with
std::runtime_error that has standard contructors from strings.
Constructor from const char* was introduced in C++11, but if the C++
standard library implementation doesn't support that constructor, the
string should be implicitly converted to std::string. Constructor from
std::string has always been there.

src/wesnoth.cpp

@@ -934,7 +934,7 @@ static void restart_process(const std::vector<std::string>& commandline)
 	GetModuleFileNameW(NULL, process_path, MAX_PATH);
 	if (GetLastError() != ERROR_SUCCESS)
 	{
-		throw std::exception("Failed to retrieve the process path");
+		throw std::runtime_error("Failed to retrieve the process path");
 	}
 
 	std::wstring commandline_str = unicode_cast<std::wstring>(utils::join(commandline, " "));