New features:

• Add configure support for --linux-android. (@cferris1000, @jasone)
• Add the --disable-syscall configure option, for use on systems that place security-motivated limitations on syscall(2). (@jasone)
• Add support for Debian GNU/kFreeBSD. (@thesam)

Optimizations:

• Add extent serial numbers and use them where appropriate as a sort key that is higher priority than address, so that the allocation policy prefers older extents. This tends to improve locality (decrease fragmentation) when memory grows downward. (@jasone)
• Refactor madvise(2) configuration so that MADV_FREE is detected and utilized on Linux 4.5 and newer. (@jasone)
• Mark partially purged arena chunks as non-huge-page. This improves interaction with Linux's transparent huge page functionality. (@jasone)

Bug fixes:

• Fix size class computations for edge conditions involving extremely large allocations. This regression was first released in 4.0.0. (@jasone, @ingvarha)
• Remove overly restrictive assertions related to the cactive statistic. This regression was first released in 4.1.0. (@jasone)
• Implement a more reliable detection scheme for os_unfair_lock on macOS. (@jszakmeister)