Check for missing pdftoppm when poppler installed with --disable-splash-output

When I upgraded to poppler 0.24.5, pdftoppm was not compiled because the
script had --disable-splash-output set for some reason.

For OS X Homebrew the solution is:
brew uninstall poppler
brew install poppler --with-splash-output
Add command line option to skip pages that contain font data

If a page contains font data, the script would abort, unless -f was given,
in which case it would use pdftoppm to rasterize the font into a bitmap
and then attempt to OCR it. -f is almost certainly not what users want
unless they want to debug OCR or something.

If a PDF already has fonts it either was OCR'd already, or it is
a composite file containing, for example, some scanned documents appended
to a text report.  In the latter case, this -s option provides OCR on
pages that don't have it without changing those that do, and if a PDF
was completely OCRed it will be converted to PDF/A.  In batch jobs with
a mix of OCR and non-OCR the implicit conversion to PDF/A is also useful.
Fix call to readlink on OS X

readlink -f is a GNU coreutils extension, so not available on OS X and
other platforms.
Merge branches 'feature/readlink-osx' and 'feature/keep-text-pages' into develop

Conflicts:
	OCRmyPDF.sh