Replace the DEBUG_DISABLE_PDFCOMPRESSION ifdef with a
VCL_DEBUG_DISABLE_PDFCOMPRESSION environment variable. The less build
configurations we have, the better, I guess.

Also remove the COMPRESS_PAGES define, which was exactly defined when
DEBUG_DISABLE_PDFCOMPRESSION wasn't.

Change-Id: Iaa5530317432e59dee0d3ea2cb606661e8283835