Add --disable-llvm-backend/--enable-llvm-backend build-time options, and move llvm.c to src/backends/llvm/ subdir

This allows the library to be built and tested with a non-default
version of CPython, e.g.

scons bindings=python python=python3.6
scons bindings=python python=python3.6 testpython

These have no effect in Python 3.x, they are the default. Enabling them
in Python 2.x, enabling them in Python 2.x allows single source
compatiblity.

To homogenize the file and allow keys/strings to be searched easily.

We disable:
- the tests (which require glib) although they can be
  reactivated with the `--tests` command line flag
- shared library (lack of export symbol declarations
  means that although it can be built, no symbol is
  exported and therefore it can't be used)

The `install` target installs the library and headers
under the `build` folder, because it's a traditional practice
to move libraries to a central location on Windows, unless
you are using cygwin. In which case pass `prefix` to the
command line.

We adapt tools\windows\build_examples.bat to take the library
that is built using scons or using tools\windows\build.bat

We will need to use the environment's CC variable to make decisions
that apply to windows compiling and linking. Therefore we move some
existing sections below the set up of env["CC"] and env["CXX"]

Scons now doesn't throw away /home/stewart/.nix-profile/lib/pkgconfig:/nix/var/nix/profiles/default/lib/pkgconfig:/run/current-system/sw/lib/pkgconfig, so NixOS can build it