Ref: nodejs/node#30339

This is to workaround previous (to v14) versions of node
and a compiler bug affected by those versions, as follows:

- v8 (ships with D190508), requires -U_VARARG_EXT_ and no-opt
- v12 (ships with D191122), requires no-opt
- v14 (ships with D191122), no workaround needed.

Find Java's lib_dir to link to libjvm.x, instead of assuming
users have their LDFLAGS env variable set to that path.

zos: rebase and apply custom compile flags to support Node v8, v12 and v14

The destructor InstanceMethodCallBaton() calls m_javaObject->Unref()
which aborts when m_javaObject is NULL.

Affects at least `npm test` from `node-jdbc` npm, which aborts in
test `testcreatetable`:

(abort) [0x26C7C640]
(__cxa_end_catch) [0x2E2CC414]
(InstanceMethodCallBaton::~InstanceMethodCallBaton()+0x140) [0x2E2CD2A4]
(Nan::AsyncWorker::Destroy()@AF61_58+0x28) [0x2E2CCE4C]
(Nan::AsyncWorker::Destroy()+0x18) [0x2E2C9F12]
(Nan::AsyncExecuteComplete(uv_work_s*)+0x4e) [0x27DB5430]
...

src: verify pointer before calling its method