external: manually apply GCC 4.8 support patch on json.hpp.

This allows us to build the TinyGltfImporter on GCC 4.8. It was disabled
until now on Linux because the Travis builds were using GCC 4.7 and
json.hpp required 4.9, but now that we're dropping 4.7 support, it makes
sense to go an extra step and have this built under 4.8 as well.

Taken from nlohmann/json#1257, applied the part
that touches single_include/nlohmann/json.hpp skipping the compiler
check on top (which doesn't apply) and doing the change by hand.

I could also update to json.hpp 3.3 / 3.4, but that version is 700 kB
(vs 480 kB for this one) and I have no reason to use that. I also hope
I'll never need to update this thing again.

src/MagnumExternal/TinyGltf/json.hpp

@@ -63,7 +63,7 @@ SOFTWARE.
         #error "unsupported Clang version - see https://github.com/nlohmann/json#supported-compilers"
     #endif
 #elif defined(__GNUC__) && !(defined(__ICC) || defined(__INTEL_COMPILER))
-    #if (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__) < 40900
+    #if (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__) < 40800
         #error "unsupported GCC version - see https://github.com/nlohmann/json#supported-compilers"
     #endif
 #endif
@@ -11832,6 +11832,23 @@ class basic_json
         return {it, res.second};
     }
 
+    /// helper for insertion of an iterator (supports GCC 4.8+)
+    template<typename... Args>
+    iterator insert_iterator(const_iterator pos, Args&& ... args)
+    {
+        iterator result(this);
+        assert(m_value.array != nullptr);
+
+        auto insert_pos = std::distance(m_value.array->begin(), pos.m_it.array_iterator);
+        m_value.array->insert(pos.m_it.array_iterator, std::forward<Args>(args)...);
+        result.m_it.array_iterator = m_value.array->begin() + insert_pos;
+
+        // For GCC 4.9+ only, this could become:
+        // result.m_it.array_iterator = m_value.array->insert(pos.m_it.array_iterator, cnt, val);
+
+        return result;
+    }
+
     /*!
     @brief inserts element
 
@@ -11866,9 +11883,7 @@ class basic_json
             }
 
             // insert to array and return iterator
-            iterator result(this);
-            result.m_it.array_iterator = m_value.array->insert(pos.m_it.array_iterator, val);
-            return result;
+            return insert_iterator(pos, val);
         }
 
         JSON_THROW(type_error::create(309, "cannot use insert() with " + std::string(type_name())));
@@ -11919,9 +11934,7 @@ class basic_json
             }
 
             // insert to array and return iterator
-            iterator result(this);
-            result.m_it.array_iterator = m_value.array->insert(pos.m_it.array_iterator, cnt, val);
-            return result;
+            return insert_iterator(pos, cnt, val);
         }
 
         JSON_THROW(type_error::create(309, "cannot use insert() with " + std::string(type_name())));
@@ -11983,12 +11996,7 @@ class basic_json
         }
 
         // insert to array and return iterator
-        iterator result(this);
-        result.m_it.array_iterator = m_value.array->insert(
-                                         pos.m_it.array_iterator,
-                                         first.m_it.array_iterator,
-                                         last.m_it.array_iterator);
-        return result;
+        return insert_iterator(pos, first.m_it.array_iterator, last.m_it.array_iterator);
     }
 
     /*!
@@ -12030,9 +12038,7 @@ class basic_json
         }
 
         // insert to array and return iterator
-        iterator result(this);
-        result.m_it.array_iterator = m_value.array->insert(pos.m_it.array_iterator, ilist.begin(), ilist.end());
-        return result;
+        return insert_iterator(pos, ilist.begin(), ilist.end());
     }
 
     /*!