How do I "conditionally" include libraries and declare global objects of them?
const bool useLCD = true;


should be:

#define useLCD 1




Here is a complete working example of how to use preprocessor macros for conditional compilation:

#define USE_A 1
// #define USE_B 1

#if (USE_A)
int a = 4;
#endif

#if (USE_B)
int b = 6;
#endif

void setup() {
    Serial.begin(115200);
    Serial.print(F("The result is: "));
#if (USE_A)
    Serial.println(a * 2);
#endif
#if (USE_B)
    Serial.println(b * 2);
#endif
}

void loop() {
}