Make wallclock_c compatible with native Windows C library

This change is necessary to build Quantum ESPRESSO on Windows using a native (non-MSYS/Cygwin) toolchain such as Intel Fortran + MSVC.

The QE repo is directly using the 0.1.0 tag at the moment, but getting this change into a branch is the necessary first step to get QE to use it. The master branch seems to me to be the closest existing candidate, but perhaps QE might prefer having a dedicated branch for 0.1.x (0.1.0 plus this fix).