Fix linking errors on some compilers.

Fix linking errors on some compilers.

Let's be explicit about variables with C language linkage [dcl.link]:
- 'type x;' declares and defines C++ variable 'x'.
- 'extern "C" type x;' declares 'x' (with C linkage).
- 'extern "C" { type x; }' declares and defines 'x'.
The C++ standard allows mixed declarations as long as they denote the
same entity, but some compilers (e.g. CLang) don't seem to comply.
It doesn't hurt to be explicit and use extern "C" throughout for the
affected variables.

Reported by Philip Huffman <philhuffman56@icloud.com>.