Added googlemock building instructions
Added googlemock building instructions

The googlemock tools are already present, so why not make use of
it. It can be usefull for testing callbacks.

amitdo/tesseract,jbarlow83/tesseract,tesseract-ocr/tesseract,UB-Mannheim/tesseract,UB-Mannheim/tesseract,amitdo/tesseract,UB-Mannheim/tesseract,UB-Mannheim/tesseract,UB-Mannheim/tesseract,stweil/tesseract,stweil/tesseract,jbarlow83/tesseract,tesseract-ocr/tesseract,amitdo/tesseract,jbarlow83/tesseract,tesseract-ocr/tesseract,stweil/tesseract,jbarlow83/tesseract,stweil/tesseract,jbarlow83/tesseract,amitdo/tesseract,stweil/tesseract,amitdo/tesseract,tesseract-ocr/tesseract,tesseract-ocr/tesseract