Add extra CXXFLAGS flags set by user; can be -D CXX_EXTRA_FLAGS or environment variable

- Command line -D option overrides environment variable
- Useful for suppressing transient upstream warnings in dependencies, like Qt
Add support for PYTHONSTARTUP-like environment variable, PYQGIS_STARTUP

- Different than user's 'startup.py,' since it is loaded just after Py_Initialize, specifically from env variable
- Useful for cleaning sys.path, e.g. isolating/loading the initial environ without requiring a virt env, like homebrew or MacPorts installs on Mac
Add unit test for PYQGIS_STARTUP environment variable

- Make PyQgsAppStartup test more general and PEP8 compliant