error: ‘NULL’ was not declared in this scope

Question

NULL is not a keyword. It's an identifier defined in some standard headers. You can include

#include <cstddef>

To have it in scope, including some other basics, like std::size_t.

GCC is taking steps towards C++11, which is probably why you now need to include cstddef in order to use the NULL constant. The preferred way in C++11 is to use the new nullptr keyword, which is implemented in GCC since version 4.6. nullptr is not implicitly convertible to integral types, so it can be used to disambiguate a call to a function which has been overloaded for both pointer and integral types:

void f(int x);
void f(void * ptr);

f(0);  // Passes int 0.
f(nullptr);  // Passes void * 0.

NULL isn't a keyword; it's a macro substitution for 0, and comes in stddef.h or cstddef, I believe. You haven't #included an appropriate header file, so g++ sees NULL as a regular variable name, and you haven't declared it.

To complete the other answers: If you are using C++11, use nullptr, which is a keyword that means a void pointer pointing to null. (instead of NULL, which is not a pointer type)