Go Jsonnet library and _Complex

[brettviren]

Lynn Garren found compiling ubsim against WCT with Clang (c14) gives

In file included from /home/garren/scratch/larsoft/uboone/dev5/srcs/ubsim/ubsim/DetSim/MicroBooNEWireCell/ReweightedDepoTransform.cxx:17:
In file included from /cvmfs/larsoft.opensciencegrid.org/products/wirecell/v0_24_1b/Linux64bit+3.10-2.17-c14-prof/include/WireCellUtil/Persist.h:31:
cgo-gcc-export-header-prolog:23:15: error: '_Complex' is a C99 extension [-Werror,-Wc99-extensions]
typedef float _Complex GoComplex64;
              ^
cgo-gcc-export-header-prolog:24:16: error: '_Complex' is a C99 extension [-Werror,-Wc99-extensions]
typedef double _Complex GoComplex128;

The problem is actually inside the C API to the Go library providing the Jsonnet parser and is a little dump because the API does not use the offending types.

Besides pushing this upstream, it should be possible to fix by simply removing the include "libjsonnet.h" entirely from Persist.h and in Persist.cxx add declaration of the subset of the API we actually use.

[brettviren]

Lynn reports that compiler flags are not enough to dodge this error so we should fast track.

We should back port this to 0.24.x in addition to making a 0.25.1