Add support for QNX

[chachoi]

Modifications:

• Change __QNXNTO__ to __QNX__ because __QNXNTO__ is deprecated
• In xmltest.cpp, do not print nullptr because it would crash in QNX
• Install tests for QNX

Build and test instuction for QNX:
https://github.com/chachoi-world/qnx-ports/blob/main/tinyxml2/README.md

Thanks.

[leethomason]

• The "install" section for QNX is above the rest of the installs - and does QNX really need something unique there?
• I don't think any system can print null strings. The changes to test looks like it may be hiding a bug

[chachoi]

Hi, @leethomason

Thanks for responding to my pull request.

The "install" section for QNX is above the rest of the installs - and does QNX really need something unique there?

Yes, QNX is cross compiled from a Ubuntu machine, so xmltest and resources have to be installed to the QNX SDP (Software Development Platform) which is like an SDK for embedded targets so that users would be able to pick up the built xmltest and resources and put them on an embedded target to test.

I don't think any system can print null strings. The changes to test looks like it may be hiding a bug

I tested this sample program on my Ubuntu Linux machine, and I was able to print a null pointer:

#include <stdio.h>

int main() {
    const char* ptr = NULL;
	printf ("test %s\n", ptr);
}

The output is test (null).

When I run the same program in QNX I get:

Process 1302562 (a.out) terminated SIGSEGV code=1 fltno=11 ip=00000035fabe9c58 mapaddr=0000000000072c58 ref=0000000000000000
Memory fault (core dumped)

xmltest for Ubuntu successfully prints:

[pass] XMLDocument::Value() returns null? [(null)][(null)]

whereas QNX crashes becuase it cannot print null pointers.