You need to link with the math library with -lm:
gcc -o sphere sphere.c -lm
The error you are seeing: error: ld returned 1 exit status is from the
linker ld (part of gcc that combines the object files) because it is
unable to find where the function pow is defined.
Including math.h brings in the declaration of the various functions and
not their definition. The def is present in the math library libm.a.
You need to link your program with this library so that the calls to
functions like pow() are resolved.