Fix build for ocrd_tesserocr (fixes issue #420)

`make all` did not build ocrd_tesserocr if it was started without a
virtual Python environment because ocrd_tesserocr requires a venv
for building.

Signed-off-by: Stefan Weil <sw@weilnetz.de>

Makefile

@@ -489,7 +489,7 @@ TESSTRAIN_EXECUTABLES += $(BIN)/text2image
 TESSTRAIN_EXECUTABLES += $(BIN)/unicharset_extractor
 TESSTRAIN_EXECUTABLES += $(BIN)/wordlist2dawg
 $(call multirule,$(OCRD_TESSEROCR)): ocrd_tesserocr $(BIN)/ocrd
-	$(MAKE) -C $< install # install-tesseract-training
+	. $(ACTIVATE_VENV) && $(MAKE) -C $< install # install-tesseract-training
 
 endif
 clean: clean-tesseract