How can I test if preprocessor symbol is #define'd but has no value?

Using C++ preprocessor directives, is it possible to test if a preprocessor symbol has been defined, but doesn't have any value? Something like that:

#define MYVARIABLE
#if !defined(MYVARIABLE) || #MYVARIABLE == ""
... blablabla ...
#endif

The reason why I am doing it is because the project I'm working on is supposed to take a string from the environment through /DMYSTR=$(MYENVSTR), and this string might be empty. I want to make sure that the project fails to compile if the user forgot to define this string.

Ssome macro magic:

#define DO_EXPAND(VAL)  VAL ## 1
#define EXPAND(VAL)     DO_EXPAND(VAL)

#if !defined(MYVARIABLE) || (EXPAND(MYVARIABLE) == 1)

Only here if MYVARIABLE is not defined
OR MYVARIABLE is the empty string

#endif

Note if you define MYVARIABLE on the command line, the default value is 1:

g++ -DMYVARIABLE <file>

Here the value of MYVARIABLE is the empty string:

g++ -DMYVARIABLE= <file>

The quoting problem solved:

#define DO_QUOTE(X)        #X
#define QUOTE(X)           DO_QUOTE(X)

#define MY_QUOTED_VAR      QUOTE(MYVARIABLE)

std::string x = MY_QUOTED_VAR;
std::string p = QUOTE(MYVARIABLE);