How to determine the source of constants in libraries?
The name TC_CTRLA_PRESCALER_DIV1 indicates this is a macro. A trick for quickly finding the exact location of a macro definition is:


File > Preferences > Compiler Warnings > All > OK
In your sketch, add a #define directive for the macro name that definitely changes the definition:


#define TC_CTRLA_PRESCALER_DIV1 foobar

Sketch > Verify/Compile
After the compilation finishes, examine the compiler output in the black console window at the bottom of the Arduino IDE window (you will need to scroll up to see it all) for a "redefined" warning. The warning will state the path of the original definition:


    C:\Users\per\AppData\Local\Temp\arduino_modified_sketch_804921\sketch_aug05a.ino:1:0: warning: "TC_CTRLA_PRESCALER_DIV1" redefined [enabled by default]

   #define TC_CTRLA_PRESCALER_DIV1 foobar

   ^

  In file included from C:\Users\per\AppData\Local\Arduino15\packages\arduino\tools\CMSIS-Atmel\1.1.0/CMSIS/Device/ATMEL/samd21/include/samd21g18a.h:268:0,

                   from C:\Users\per\AppData\Local\Arduino15\packages\arduino\tools\CMSIS-Atmel\1.1.0/CMSIS/Device/ATMEL/samd21/include/samd21.h:69,

                   from C:\Users\per\AppData\Local\Arduino15\packages\arduino\tools\CMSIS-Atmel\1.1.0/CMSIS/Device/ATMEL/samd.h:105,

                   from C:\Users\per\AppData\Local\Arduino15\packages\arduino\tools\CMSIS-Atmel\1.1.0/CMSIS/Device/ATMEL/sam.h:470,

                   from C:\Users\per\AppData\Local\Arduino15\packages\arduino\hardware\samd\1.6.19\cores\arduino/Arduino.h:48,

                   from C:\Users\per\AppData\Local\Temp\arduino_build_309126\sketch\sketch_aug05a.ino.cpp:1:

  C:\Users\per\AppData\Local\Arduino15\packages\arduino\tools\CMSIS-Atmel\1.1.0/CMSIS/Device/ATMEL/samd21/include/component/tc.h:113:0: note: this is the location of the previous definition

   #define TC_CTRLA_PRESCALER_DIV1     (TC_CTRLA_PRESCALER_DIV1_Val   &lt;&lt; TC_CTRLA_PRESCALER_Pos)

   ^