Log Message:
squid6: update to 6.12

Switch to github releases as per Squid project's new release strategy.
Use USE_CXX_FEATURES to specify c++17 instead of USE_LANGUAGES.
Remove superseded c++11 assertion from options.mk

Changes from 6.11:
- Fix validation of Digest auth header parameters
- Fix build with libtool v2.4 + automake v1.17
- Use git to extract default build-info when enabled
- ... and a minor source-maintenance portability fix