Readd and rework convertes/pdf2djvu

This port was deleted due to no public distfiles. Now upstream distributes
tarballs on GitHub [1].

• Update to 0.9.12
• Add USES=autoreconf:build and a bootstrap file; define the target pre-configure
• Make the manual within the post-build target
• Add USE_CXXSTD=c++11
• Add options COLOR_QUANTIZATION, NLS, MULTITHREADING, XMP
• Add LICENSE_FILE
• Patch tools/install-manpages for proper echoing (no functional change)
• Remove the port name from COMMENT to follow the guidelines of the handbook
• Strip the executable within the post-install target
• Submitter is the new MAINTAINER

[1] https://github.com/jwilk/pdf2djvu

PR: 236299
Submitted by: Samy Mahmoudi <samy.mahmoudi@gmail.com>