Soma 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);