Move cold compiler on master to 4.13.1

[dra27]

• Move the cold compiler to 4.13 in order to have a version which doesn't suffer from the SIGSTKSZ problem with glibc 2.34
• Bump the src_ext dependencies to latest versions while here
• In 4.13, the FlexDLL sources needed to be available before configure is run, so move the extraction further up
• The Cygwin32 build is really struggling to build a compiler and run the testsuite at once - the final commit tweaks the build processes so that instead of running make world followed by make opt.opt we do the fractionally faster make world.opt (if the secondary compiler were being built, it does make world followed by make opt).

[dra27]

This one's passed on AppVeyor on my fork

[dra27]

Merging now to unblock the base image builder!