Document minimum clang version on Linux

[Paul Hargrove]

We need to document the minimum clang version that works (or a conservative approximation).

Based on our CI testing w/ GASNet, I know 4.0.1 works.

Based on my comments today in issue #7, I know that 3.8.1 works.

HOWEVER, in both cases the clang++ was built with gcc-7.2.0 and therefore is using the STL/libstdc++ from gcc-7.2.0. So, we may need to document a clang front-end version and mention that it needs a minimum libstdc++ (hopefully the one from whichever minimum gcc version we document).

[Paul Hargrove]

I can add Clang 3.7.1 (still built w/ gcc-7.2.0) to the "WORKS" list.
I am not promising to get a tight bound, but as long as I don't hit problems building clang I will continue.

[Paul Hargrove]

I find Clang 3.7.0 works as well.
Meanwhile, Clang 3.6.2 failed to build backend.cpp (I can provide output if desired).

So: On Linux one needs Clang 3.7.0 built with a "new-enough" libstdc++.
I am currently assuming that "new-enough" likely means gcc-5.1.0 but will test that assumption.

[Paul Hargrove]

Clang 3.7.0 built with gcc-5.1.0 works (as hoped).

Dan has reminded me by email that all of this is for the current develop branch, and subject to change as the implementation grows and potentially encompasses more of c++'s less widely-implemented features.

I have reached back as far in clang as possible, and as far back in gcc's as the first libstdc++ version Amir claims is sufficient. So I am resolving this "Linux/x86-64 with clang-3.7.0 or newer built to use the libstd++ from gcc-5.1.0 or newer").

[Paul Hargrove]

Linux/x86-64 with clang-3.7.0 or newer built to use the libstd++ from gcc-5.1.0 or newer.