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inline half CustomDotClamped ( half3 a, half3 b ){ 
 #if (SHADER_TARGET < 30)
    return saturate(dot(a,b));
 #else
    return max(0.0h, dot(a,b));
#endif
}

inline half CustomLambertTerm ( half3 normal, half3 lightDir ) {
  return smoothstep(0.0,0.05f, CustomDotClamped (normal, lightDir)); //I only added 1 character but the design of this site prevents 1 character edits hence I had to also add this comment :(
}

inline half CustomDotClamped ( half3 a, half3 b ){ 
 #if (SHADER_TARGET < 30)
    return saturate(dot(a,b));
 #else
    return max(0.0h, dot(a,b));
#endif
}

inline half CustomLambertTerm ( half3 normal, half3 lightDir ) {
  return smoothstep(0.0,0.05f, CustomDotClamped (normal, lightDir));
}

inline half CustomDotClamped ( half3 a, half3 b ){ 
 #if (SHADER_TARGET < 30)
    return saturate(dot(a,b));
 #else
    return max(0.0h, dot(a,b));
#endif
}

inline half CustomLambertTerm ( half3 normal, half3 lightDir ) {
  return smoothstep(0.0,0.05f, CustomDotClamped (normal, lightDir);
}

inline half CustomDotClamped ( half3 a, half3 b ){ 
 #if (SHADER_TARGET < 30)
    return saturate(dot(a,b));
 #else
    return max(0.0h, dot(a,b));
#endif
}

inline half CustomLambertTerm ( half3 normal, half3 lightDir ) {
  return smoothstep(0.0,0.05f, CustomDotClamped (normal, lightDir)); //I only added 1 character but the design of this site prevents 1 character edits hence I had to also add this comment :(
}

Updated for Unity 5.5 : WORKS

Note : Some of the functions used here have been deprecated in Unity 5.5.

Note that some of the functions used here have been deprecated in Unity 5.5, but I have tested it to work.