aqua - Include support for the Mac OS X Aqua (Carbon/Cocoa) GUI
debug - Enable extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Backtraces
doc - Add extra documentation (API, Javadoc, etc). It is recommended to enable per package instead of globally
epub - Enable support for reading EPUB ebook metadata
exif - Add support for reading EXIF headers from JPEG and TIFF images
ffmpeg - Enable ffmpeg/libav-based audio/video codec support
kernel_linux - KERNEL setting for system using the Linux kernel
libav - Prefer libav over ffmpeg whenever both are supported
office - Enable support for reading ODF and OOXML metadata
pdf - Add general support for PDF (Portable Document Format), this replaces the pdflib and cpdflib flags
