Set C++11 requirements

[Flamefire]

Due to lexical_cast the library requires C++11 which it doesn't advertise making dependents fail in C++03 CI

This sets the requirements manually (atomic and lexical_cast is [used] header-only so we don't get them automatically)

I had an alternative which I want to mention here which basically removes the dependency on lexical_cast:

diff --git a/CMakeLists.txt b/CMakeLists.txt
index d724ea36..38c137e8 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -83,7 +83,6 @@ target_link_libraries(boost_thread

   PRIVATE
     Boost::algorithm
-    Boost::lexical_cast
 )

 target_compile_definitions(boost_thread
diff --git a/src/pthread/thread.cpp b/src/pthread/thread.cpp
index c17aca2c..6d75a5ec 100644
--- a/src/pthread/thread.cpp
+++ b/src/pthread/thread.cpp
@@ -35,9 +35,9 @@

 #include <boost/algorithm/string/split.hpp>
 #include <boost/algorithm/string/trim.hpp>
-#include <boost/lexical_cast.hpp>

 #include <fstream>
+#include <sstream>
 #include <string>
 #include <set>
 #include <vector>
@@ -47,6 +47,17 @@ namespace boost
 {
     namespace detail
     {
+        template<typename Target>
+        Target classic_convert(const std::string& src)
+        {
+            std::istringstream s(src);
+            s.imbue(std::locale::classic());
+            Target res;
+            if(s >> res)
+                return res;
+            throw std::bad_cast("Cannot convert " + src);
+        }
+
         thread_data_base::~thread_data_base()
         {
             for (notify_list_t::iterator i = notify.begin(), e = notify.end();
@@ -547,12 +558,12 @@ namespace boost
                 boost::trim(value);

                 if (key == physical_id) {
-                    current_core_entry.first = boost::lexical_cast<unsigned>(value);
+                    current_core_entry.first = classic_cast<unsigned>(value);
                     continue;
                 }

                 if (key == core_id) {
-                    current_core_entry.second = boost::lexical_cast<unsigned>(value);
+                    current_core_entry.second = classic_cast<unsigned>(value);
                     cores.insert(current_core_entry);
                     continue;

Might be worth a consideration as I'm not sure lexical_cast (IIRC localized conversion) is correct here.