Fix #239 by explicitly declaring Jsonnet API functions used

[brettviren]

At least, perhaps. I could not reproduce the problem. This explicitly declares the Jsonnet API functions instead of including the header that is causing FNAL problems

[HaiwangYu]

Hi Brett, I got an error when trying to compile with g++ (GCC) 9.3.0

g++: error: unrecognized command line option '-Wc99-extensions'; did you mean '-fms-extensions'?

Is that flag needed?
Other than that, I can build and run tests with out issue.

[brettviren]

Arg, darn! Sorry, that was meant to be a temporary addition to try to get clang to reproduce the problem that Lynn reported. I'll refresh the three branches.

[brettviren]

@HaiwangYu

It was perhaps fortuitous to take another pass as we were missing many compiler warnings in WCT 0.24.x (still many in boost we can't fix).

This PR branch and the 0.24.x and 0.25.x branches have -Wc99-extensions removed. In addition I updated the release branches a bit more:

• 0.24.x added fixes for many compiler warnings found by clang and one failed test due to clang 14 changing how it demangles C++ symbols.
• 0.25.x remove one compiler warning GCC found.

I believe I've tested all 3 branches X 2 compilers. Hopefully I didn't miss a matrix element.