Fails to build with `python setup.py build`

Unlike other Python packages, this one fails to build:

```

  running build

  running build_py

  creating build

  creating build/lib.mingw_x86_64_clang-cpython-311

  creating build/lib.mingw_x86_64_clang-cpython-311/pybase64

  copying src/pybase64/_fallback.py -> build/lib.mingw_x86_64_clang-cpython-311/pybase64

  copying src/pybase64/_license.py -> build/lib.mingw_x86_64_clang-cpython-311/pybase64

  copying src/pybase64/_version.py -> build/lib.mingw_x86_64_clang-cpython-311/pybase64

  copying src/pybase64/__init__.py -> build/lib.mingw_x86_64_clang-cpython-311/pybase64

  copying src/pybase64/__main__.py -> build/lib.mingw_x86_64_clang-cpython-311/pybase64

  copying src/pybase64/py.typed -> build/lib.mingw_x86_64_clang-cpython-311/pybase64

  running build_ext

  running cmake '--version'

  cmake version 3.28.1

  

  CMake suite maintained and supported by Kitware (kitware.com/cmake).

  getting cmake extra config

    machine: amd64

    platform: mingw_x86_64_clang

    plat_name: mingw_x86_64_clang

    ARCHFLAGS: None

    CC: clang

    CFLAGS: -march=nocona -msahf -mtune=generic -O2 -pipe -Wp,-D_FORTIFY_SOURCE=2 -fstack-protector-strong

    LDFLAGS: -pipe

    sysconfig CC: clang

    sysconfig CCSHARED: 

    sysconfig CFLAGS: -DNDEBUG -g -fwrapv -O3 -Wall -march=nocona -msahf -mtune=generic -O2 -pipe -Wp,-D_FORTIFY_SOURCE=2 -fstack-protector-strong -O3 -march=nocona -msahf -mtune=generic -O2 -pipe -Wp,-D_FORTIFY_SOURCE=2 -fstack-protector-strong -O3

    sysconfig BASECFLAGS: 

    sysconfig OPT: -DNDEBUG -g -fwrapv -O3 -Wall

    sysconfig LDFLAGS: -pipe -pipe

  building 'pybase64._pybase64' extension

  creating build/temp.mingw_x86_64_clang-cpython-311

  creating build/temp.mingw_x86_64_clang-cpython-311/src

  creating build/temp.mingw_x86_64_clang-cpython-311/src/pybase64

  clang -DNDEBUG -g -fwrapv -O3 -Wall -march=nocona -msahf -mtune=generic -O2 -pipe -Wp,-D_FORTIFY_SOURCE=2 -fstack-protector-strong -O3 -march=nocona -msahf -mtune=generic -O2 -pipe -Wp,-D_FORTIFY_SOURCE=2 -fstack-protector-strong -O3 -march=nocona -msahf -mtune=generic -O2 -pipe -Wp,-D_FORTIFY_SOURCE=2 -fstack-protector-strong -D__USE_MINGW_ANSI_STDIO=1 -DBASE64_STATIC_DEFINE=1 -Ibase64/include/ -Ibase64/lib/ -I.base64_build -ID:/M/msys64/clang64/include/python3.11 -c src/pybase64/_pybase64.c -o build/temp.mingw_x86_64_clang-cpython-311/src/pybase64/_pybase64.o

  src/pybase64/_pybase64.c:4:10: fatal error: 'config.h' file not found

      4 | #include <config.h>

        |          ^~~~~~~~~~

  1 error generated.

  warning: BuildExt: building extension "pybase64._pybase64" failed: command 'D:\\M\\msys64\\clang64\\bin/clang.exe' failed with exit code 1

```

It expects base64 to be in the directory (`-Ibase64/include/`) but setuptools creates a temporary build directory.

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