First time creating a user library.  Here are the salient bits:
in mySketch.ino:

#define DEBUG
#include <mylib.h>

in mylib.h:
#ifdef DEBUG
     #undef UART
     const int RX_PIN = 10;
     const int TX_PIN = 11;
#else
     #define UART
#endif
     
#ifdef UART
     #define SERIAL Serial
#else
     #include <SoftwareSerial.h>
     SoftwareSerial mySerial(RX_PIN, TX_PIN);
     #define SERIAL mySerial
#endif

in mylib.cpp:
void func()
{
#ifndef UART
     pinMode(RX_PIN, INPUT);
     pinMode(TX_PIN, OUTPUT);
#endif

     SERIAL.begin( BAUD_RATE );
...
}

Here's the error I get:
Error linking for board Arduino/Genuino Uno
mylib.cpp.o (symbol from plugin)*: In function mySerial
Debug build failed for project 'MySketch'
(.text+0x0)*: multiple definition of mySerial
MySketch.cpp.o (symbol from plugin)*: (.text+0x0): first defined here
 
collect2.exe*: error: ld returned 1 exit status