New `-Werror=undef` issues along the lines of #196

[axxel]

The current HEAD fails to compile with g++ using -Werror=undef -std=c++17 with a couple errors like:

ctre.hpp:388:21: error: "__cpp_char8_t" is not defined, evaluates to 0 [-Werror=undef]
  388 |                 #if __cpp_char8_t

The following one-liner fixes the problem:

sed -i -E "s/#if (__cpp_[^ ]*)/#if defined \1 \&\& \1/g" ctre.hpp

See also this commit.