Currently, the pdf-mime-data check just fails for me with the following
output:

    sh: 1: pdfimages: not found
    pdf-mime-data: FAIL

pdf-mime-data.log contains:

    pdfimages failed with exit status 32512

Since I do not have pdfimages installed... yeah.

This commit "fixes" that problem by skipping the test if pdfimages is
not available. No idea if it would pass if it were available, but I do
not feel like installing pdfimages just to test.

Signed-off-by: Uli Schlachter <psychon@znc.in>