Latest code fails to build all

[stweil]

There are several issues with the current git master:

• make all tries to compile opencv-python although that is only needed for some architectures like ARM. Fix: PR Don't add opencv-python to OCRD_MODULES by default #39.
• make all fails to build cor-asv-fst.
• make all -k does not continue with other modules after the failure with cor-asv-fst. Fix: PR fix f8af582: avoid shared build rule (only share recipe) #38.
• make all with a restricted list of modules (no cro-asv-fst) does not build ocrd-tesserocr-*. Fix: PR fix f8af582: avoid shared build rule (only share recipe) #38.

[stweil]

Related: issue #36.

[kba]

#38 and #39 should fix this.