All 3Ps include dirs are treated as SYSTEM, this will stop compilers
stop emitting warnings from those files and greatly helps having high
warning levels and not being swamped by warnings that come from
external libraries.

- Targets now define the appropriate c++ defines, no more hand magic needed for llstartup.cpp
- Switch cmakeflage to USE_xxx rather than xxx as it was before. I personally find the USE_X notion more intuitive and it follows how KDU is used.
- To be backward compatible OPENAL/FMODSTUDIO flag will be mapped to USE_OPENAL/USE_FMODSTUDIO

- Fix usage of bugsplat::bugsplat by using ll::bugsplat
- Use bugsplat define by importing target not by using hand crafted magic

variables.

for example), the define it sets it used. The vrabiables it did set are
unused.

There is a case to probe for pulseaudio and this is the Linux volume
catcher. But for that it is only necessary to detect the system include
to get a few defines and functions

what's with that, maybe it gets set by LLs teamcity builds?

not make it go all crazy.

operation system needs as dependencies on OS gui libraries.

Only try copy voice files from slvoice_src_dir if that variable is set (seems to be the case for Windows)

LEGACY_STDIO_LIBS (was only used for Windows)
PTHREAD_LIBRARY (only Linux)
LLDATABASE_LIBRARIES (that one was supposed for Linux, but never needed anyway)

This gets rid of the a few OS specific set and uses variables (which some even seemed mostly
duplicate like WINDOWS_LIBRARIES ans UI_LIBRARIES) and it also solves the problem of
having them to tack on every target, as of no they come as a transitive dependency from llcommon

some internal logic on the LL build farm.

Rework cmake, the original plan was to maybe be able to use conan targets with the same name (that's why 3ps had names like apr::apr),
but it's safer and saner to put the LL 3ps under the ll:: prefix.
This also allows means it is possible to get rid of that bad "if( TRAGET ...) return() endif()" pattern and rather use include_guard().