Fix c++ error caused by c99 static array size

[twinstar6980]

Due to the changes in PR 593, quickjs-ng cannot be compiled in C++ mode.

C++ does not support the T val[static n] array syntax, which will cause the compiler to report an error: error: static array size is a C99 feature, not permitted in C++(clang).

This commit avoids entering the static n branch in C++ mode.

[saghul]

How come our c++ CI doesn't catch this? How are you building your software?

[bnoordhuis]

@twinstar6980 you can include quickjs.h in a C++ file but quickjs itself must be compiled as C, not C++

[twinstar6980]

The cpp file in my project references the cutils.h file, so it causes the error. For some reason, I referenced it in the project a long time ago.
cutils.h seems to be a private header. Inside quickjs, it will only work in C mode and will never trigger this error.

[bnoordhuis]

Right, that explains it. I'll go ahead and close this.