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

Wrapping the SHLINKFLAGS in an array rather than as a single string
caused the link flags to be passed to clang double quoted. This cuased
clang to interpret the flags as a string rather than as a flag which
caused the -install_name flag to never be passed to the linker.

This actually is only for OS X. On linux ldconfig must be run after
installing in order to update the library cache. Need to look into how
to get scons to do this for the user.

Set the proper path for the shared library so that anything linking
against it will be able to find the library.

Go, and likely other language bindings will, need access to create
new combinators. This functionality is currently located
src/parsers/parser_internal.h. Install this header to the system with
dependent headers until a better solution can be found.

Create a pkg-config file and install it to $prefix/lib/pkgconfig

Don't pass the entire environment to SCons. Only pass the PATH

Don't clober the Environment (and thus the ENV['PATH'] by redefining it
and don't link against lr by default. Only add it if uname is not Darwin