Support MacOS for M1 natively

[stweil]

Currently OCR-D cannot be built natively on MacOS for M1 processors.

This issue tries to track the known problems and solutions.

Working:

• Installation of developer tools (Xcode, Homebrew) works.
• MacOS provides Python 3.9.1.
• ocrd_all can be cloned, make all builds tesseract.

Smaller hacks required:

• Homebrew needs some smaller hacks for some packages.
• Tesseract needs fixes for training tools (not required for make all).

Failing:

• numpy fails to build: clang: error: the clang compiler does not support 'faltivec', please use -maltivec and include altivec.h explicitly.
• opencv-python-headless fails to build.
• All processors fail to build because of the missing numpy.
• Neither tensorflow nor tensorflow-gpu are available in any version.

[stweil]

Running make all PIP_OPTIONS="--pre --extra-index-url=https://digi.bib.uni-mannheim.de/pypi" -k now creates at least these processors:

venv/bin/ocrd                           venv/bin/ocrd-import                    venv/bin/ocrd-tesserocr-binarize        venv/bin/ocrd-tesserocr-segment-line
venv/bin/ocrd-dinglehopper              venv/bin/ocrd-make                      venv/bin/ocrd-tesserocr-crop            venv/bin/ocrd-tesserocr-segment-region
venv/bin/ocrd-dummy                     venv/bin/ocrd-olahd-client              venv/bin/ocrd-tesserocr-deskew          venv/bin/ocrd-tesserocr-segment-table
venv/bin/ocrd-export-larex              venv/bin/ocrd-olena-binarize            venv/bin/ocrd-tesserocr-fontshape       venv/bin/ocrd-tesserocr-segment-word
venv/bin/ocrd-fileformat-transform      venv/bin/ocrd-pagetopdf                 venv/bin/ocrd-tesserocr-recognize
venv/bin/ocrd-im6convert                venv/bin/ocrd-repair-inconsistencies    venv/bin/ocrd-tesserocr-segment