teensyduino with teensy LC: cannot include multiple files in sketch
I get the error "undefined reference to doNothing() in function loop"


The C++ compiler cannot find doNothing(). This is mainly due to the incomplete header file and mix of C and C++. 

Two things are needed to avoid this type of problem; 1) protect the file from multiple includes (in the same compile), 2) make the function visible to the C++ compiler as a C function (without name mangling).   

tst.h:

#ifndef TST_H
#define TST_H

#ifdef __cplusplus
extern "C"{
#endif

void doNothing();

#ifdef __cplusplus
} // extern "C"
#endif
#endif


Cheers!