Fix wrong environment test that breaks clang++ builds.
Fix wrong environment test that breaks clang++ builds.

g++ builds require extra flags rejected by clang++. The bug is that the
flags are actually added unconditionally. This commit fixes the
condition.

See https://github.com/tesseract-ocr/tesseract/pull/1474

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