[FIX] linux/configure.ac: Fix tesseract conditional problem.

Fixes #1503.

Using tesseract-ocr's stock pkg-config, it would produce an error due to unquoted whitespace:

$ test ! -z pkg-config --libs-only-l --silence-errors tesseract bash: test: syntax error: `-larchive' unexpected

linux/configure.ac: Use a positive test, and double-quote the $() command substitution.

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