Allow runtime/api.h to be used in C++ code
Allow runtime/api.h to be used in C++ code

Conditionally add extern "C" { ... } around the function declarations
for C++ compilers.

Closes #90, reported by vvarma.

snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball