[C/C++] make constant to be l-value.

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When compare values, try to put constant or returned value from function call into left side - make it as l-value.
This habit can prevent you from mistake - using '=' instead of '=='.
(Compiler can detect this error(mistake)!)

#define X_VAL (100)
...
if( a == X_VAL ) { ... } // This is not good!
if( X_VAL == a) { ... } // Better