restore cmake to be capable of figuring out the zig version

restore config.h and config.zig. config.h is used to detect whether we
should propagate cmake configuration information to build.zig; however
it can be overridden with -Dstatic-llvm.

fix not passing -DZIG_LINK_MODE with zig build.

when using the cmake build path, build.zig no longer tries to call
llvm-config. Instead it relies 100% on the LLVM_LIBRARIES cmake variable.

build.zig logic reworked and simplified.