Consider supporting snmalloc in Ubuntu 16.04 (GCC 5.4.0)

[anakrish]

snmalloc does not compile with the default GCC (5.4.0) on Ubuntu 16.04. This is due to the use of specific C++17 features like if constexpr.
As part of PR openenclave/openenclave@efd9311, I had to disable snmalloc build on Ubuntu 16.04 when the compiler is GCC.

Consider supporting default compilers of slightly older platforms like Ubuntu 16.04. That would make adopting snmalloc easier.

[mjp41]

Supporting a four year old compiler is not something we are resourced to do. As 18.04 LTS is out surely Open Enclave should be pushing for people to use that. Also all the security features around spectre mitigations, etc. are completely missing from GCC 5.4.0. There are clang packages for 16.04, so the snmalloc component could be built with clang, and linked in.

The core difficulty is not the if constexpr but the inline static fields. Prior to C++17 on GCC this required hideous template hacks to get the desired behaviour.

Is this actually a customer requirement for OE? We are happy to support an engineer in back-porting C++14, if someone else funds it.

[SchrodingerZhu]

@anakrish for older ubuntu release, there is a repo to get up-to-date toolchains: https://launchpad.net/~ubuntu-toolchain-r/+archive/ubuntu/test

Add this PPA should not polute the global names, for example gcc version 10 will end up with an executable called gcc-10.

[mjp41]

16.04 is no longer required for OE. Closing this.