This does the following.
1. Fixes checking if the CXX compiler works on platforms other than windows.
2. Turns the error when the CXX compiler is missing or doesn't work into a warning.
3. Adds HAVE_CC and HAVE_CXX.
4. Only adds CC and CXX to config.mk when HAVE_CC or HAVE_CXX are true.
5. Disables Qt companion, Vulkan, CXX_BUILD and NEED_CXX_LINKER if HAVE_CXX is false.
6. Explicitly errors when the CXX compiler is broken or missing and Qt or vulkan support is enabled.
7. No longer explicitly links with the CXX compiler on windows since this should no longer be needed.
This also adds the function `check_enabled` to `qb/qb.lib.sh` which
can be used to dynamically disable any libraries that require C++
support.
