Use Safe Constants Idioms for ObjFileParser::DEFAULT_MATERIAL.

Reference: "A static Class Member" in https://abseil.io/tips/140

code/AssetLib/Obj/ObjFileParser.cpp

@@ -47,7 +47,6 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #include <assimp/BaseImporter.h>
 #include <assimp/DefaultIOSystem.h>
 #include <assimp/ParsingUtils.h>
-#include <assimp/material.h>
 #include <assimp/DefaultLogger.hpp>
 #include <assimp/Importer.hpp>
 #include <cstdlib>
@@ -56,7 +55,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 namespace Assimp {
 
-const std::string ObjFileParser::DEFAULT_MATERIAL = AI_DEFAULT_MATERIAL_NAME;
+constexpr char ObjFileParser::DEFAULT_MATERIAL[];
 
 ObjFileParser::ObjFileParser() :
  m_DataIt(),

code/AssetLib/Obj/ObjFileParser.h

@@ -42,6 +42,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #define OBJ_FILEPARSER_H_INC
 
 #include <assimp/IOStreamBuffer.h>
+#include <assimp/material.h>
 #include <assimp/mesh.h>
 #include <assimp/vector2.h>
 #include <assimp/vector3.h>
@@ -141,7 +142,7 @@ class ASSIMP_API ObjFileParser {
  // because the class contains pointer to allocated memory
 
  /// Default material name
- static const std::string DEFAULT_MATERIAL;
+ static constexpr char DEFAULT_MATERIAL[] = AI_DEFAULT_MATERIAL_NAME;
  //! Iterator to current position in buffer
  DataArrayIt m_DataIt;
  //! Iterator to end position of buffer