Divert LCC compiler as a new one, instead of treating it as GNU.
Since old times, Elbrus C/C++/Fortran Compiler (LCC) by MCST has been
passing checks for GNU compilers, so it has been identified as GNU.
Now, with intent of seriously upstreaming its support, it has been
added as a separate LCC compiler, and its version displays not a
supported GCC version, but LCC version itself (e.g. LCC 1.25.19 instead
of GNU 7.3.0).
This commit adds its support for detection, and also converts basically
every check like 'is this compiler GNU?' to 'is this compiler GNU or
LCC?'. The only places where this check is untouched, is where it
regards other platforms where LCC is unavailable (primarily non-Linux),
and where it REALLY differs from GNU compiler.
Note: this transition may break software that are already ported to
Elbrus, but hardly relies that LCC will be detected as GNU; still such
software is not known.
When testing compiler modes higher than C++11 for constructs we need,
include a check for using `unique_ptr` in that mode. The PGI 18.4
compiler in some environments supports `unique_ptr` in C++11 mode
but is broken for C++14 and C++17. Check that `unique_ptr` works
in these modes before using one.
The check for C++14 and cstdio is a special case of the more general
problem of checking that the compiler's C++14 mode supports everything
we need. Rename the checks accordingly.
