Add support for MacOS and old compilers (not supporting C++11). Main patch is from Samuel Marks.

src/fulfillment_item.cpp

@@ -17,6 +17,7 @@
   along with libgourou. If not, see <http://www.gnu.org/licenses/>.
 */
 
+#include <cctype>
 #include <fulfillment_item.h>
 #include <libgourou_common.h>
 #include "user.h"
@@ -93,8 +94,12 @@ namespace gourou
     std::string FulfillmentItem::getMetadata(std::string name)
     {
 	// https://stackoverflow.com/questions/313970/how-to-convert-an-instance-of-stdstring-to-lower-case
+	#if __STDC_VERSION__ >= 201112L
 	std::transform(name.begin(), name.end(), name.begin(),
 		       [](unsigned char c){ return std::tolower(c); });
+	#else
+	std::transform(name.begin(), name.end(), name.begin(), tolower);
+	#endif
 	name = std::string("dc:") + name;
 	pugi::xpath_node path = metadatas.select_node(name.c_str());