Update to new rawtherapee release 5.7.

Quoting the webpage:
+ Film Negative tool, for easily developing raw photographs

of film negatives.

+ Support for reading “rating” tags from Exif and XMP, shown in the

File Browser/Filmstrip using RawTherapee’s star rating system.

+ Hundreds of bug fixes, speed optimizations and

raw format support improvements.

While here, simplify compiler choice and optimize the output:

• always require GCC >= 9 and binutils
• remove OPTIMIZED_CFLAGS option and always turn it on
• always require SSE2 on i386 and amd64
• turn on LTO (link-time optimization/LTCG link-time code generation (I just tried one rawtherapee-cli conversion on a Sony ARW file, and CPU user time went down from 25 to 17 s)
• add a note about lensfun data set downloads