* Use `android-environment` instead of hardcoding paths
* Remove hacks for specifying include directories and standard
library manually
* Streamline compiler options with what CMake would use under
Android
* Use C++17
sysroot and gcc-toolchain seem to be unneeded with toolchain with
platform versions in triplets.
Also unified the header search paths to use those headers from llvm
toolchain as much as possible.
This seems to fix the problem with dlopen refusing to load with "using
IE access model".