We're already using the standard find_package method to setup Qt
for the most part, so none of the manipulations of SetupQt should
be needed for system Qt.
Support 3 options:
* VIRCADIA_USE_SYSTEM_QT is set -- use system's Qt
* VIRCADIA_QT_PATH is set -- use Qt found in that dir
* Otherwise -- use downloaded Qt package
This removes these variables:
* VIRCADIA_USE_QT_VERSION
* VIRCADIA_USE_PREBUILT_QT
* HIFI_QT_BASE
* Reformat whitespace in README.md
* hifi_vckpg.py: Remove macos var/temp hack for vcpkg
* hifi_vckpg.py: Add ability to override HIFI_QT_INSTALL_ROOT dir with environment variable.
* hifi_vckpg.py: Update macos url for pre-built qt 5.12.3 on S3.
* cmake/ports/bullet3/portfile.cmake: add comment to force change in vcpkg hash.
* tools/qt-builder/README.md: updated formatting and added details about how to generate symbols for macos and pc
* tools/qt-builder: added prepare python scripts for symbols
* cmake/macros/SetupQt.cmake: Remove hardcoded var/temp qt cmake path
This change allows -DQT_CMAKE_PREFIX_PATH to function again. Previously, set(QT_CMAKE_PREFIX_PATH "$ENV{QT_CMAKE_PREFIX_PATH}") always ran, which overrode QT_CMAKE_PREFIX_PATH every time on cmake. This proposed change now only uses the environment variable if DQT_CMAKE_PREFIX_PATH is not specified.
