pzstd: use c++14 without conditions

pzstd: use c++14 without conditions

Doing this check with a direct c++ snippet is prone to portability problems:

- \043 is not portable between shells: dash expands it to #,
bash does not;

- using # directly works with make 4.3 but does not with make 4.2.

Let's just use the c++ version that covers both the code and the gtest.