Fix: Support 'old' GCC ABI

The old ABI didn't support custom allocators for standard containers. So we'll disable the use of polymorphic allocators in that case.

include/fastgltf/types.hpp

@@ -39,6 +39,13 @@
 // Utils header already includes some headers, which we'll try and avoid including twice.
 #include "util.hpp"
 
+#if defined(_GLIBCXX_USE_CXX11_ABI) && !_GLIBCXX_USE_CXX11_ABI
+// polymorphic allocators are only supported with the 'new' GCC ABI.
+// Older compilers (older than GCC 5.1) default to the old ABI and sometimes the old ABI is
+// explicitly selected on even the newest compilers, which we want to support.
+#define FASTGLTF_DISABLE_CUSTOM_MEMORY_POOL 0
+#endif
+
 #ifndef FASTGLTF_DISABLE_CUSTOM_MEMORY_POOL
 #define FASTGLTF_DISABLE_CUSTOM_MEMORY_POOL 0
 #endif