Compilation fails when `pkg-config` gives multiple arguments

`setup.py` uses the `pkgconfig` package to query the `pkg-config` database to learn the flags to use for compiling against `liblz4`. The `pkgconfig.cflags()` and `pkgconfig.libs()` functions each return a single string that might represent multiple arguments, which are then being appended to `extra_compile_args` and `extra_link_args` as though they're a single argument, resulting in link failures when the appropriate linker flags include a `-L/path/to/lib/dir`, for instance, in addition to `-llz4`.



At a minimum, this code:

```py

    if liblz4_found:

        extra_link_args.append(pkgconfig_libs('liblz4'))

        if pkgconfig_cflags('liblz4'):

            extra_compile_args.append(pkgconfig_cflags('liblz4'))

```



Should be changed to:

```py

    if liblz4_found:

        extra_link_args.extend(pkgconfig_libs('liblz4').split())

        extra_compile_args.extend(pkgconfig_cflags('liblz4').split())

```



Or, better still, instead of packing everything into `extra_link_args` and `extra_compile_args` using `pkgconfig.libs()` and `pkgconfig.cflags()`, it's more typical to use `pkgconfig.parse()`, which splits it out into separate lists to be passed for the `define_macros`, `include_dirs`, `library_dirs`, and `libraries` keyword arguments to the `Extension` constructor.

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