cmake: proper handle the extra poppler CFLAGS

Make sure to join the extra CFLAGS with a space, otherwise they are
passed as list to set_source_files_properties; also make sure to quote
the string.
gdcminfo: support poppler 0.25.1

Check for the new API of StructTreeRoot, adapting the check for a
tagged PDF accordingly. Now Catalog::getStructTreeRoot() returns NULL
if StructTreeRoot is not a dictionary.
Fixed/suppressed all clang -Windent warnings

- 1 was definately a valid complaint.
- Another was debatable, but pretty odd formatting.
- Another was a false positive, but I just added braces.
Merge branch 'release'