Release candidate 1 of lcms2-2.12

lcms2-2.12 is a maintenance release

WARNING: THIS IS A RELEASE CANDIDATE. FINAL CONTENTS OF 2.12 MAY VARY

Changes:
Added build system for fast-float plugin (see plugin documentation)
Added new build-in sigmoidal tone curve
Added XCode 12 project
Addes support for multichannel input up to 15 channels
Fix LUT8 write matrix
Fix version mess on 10/11
Fix tools & samples xgetopt
Fix warnings on different function pointers
Fix matlab MEX compilation
plugin: cleanup and better SSE detection
plugin: add lab to any on float
plugin: it can now be compiled as C++
recover PDF documentation, but try to keep it under a resonable size.
Prevent a rare but possible out-of-bounds read in postscript generator