Clarify usage of #if vs #ifdef and standardize everywhere

[lukasdreyer]

Feature request

Is your feature request related to a problem? Please describe.
It is not clear to me, whether #if T8_ENABLE_DEBUG or #ifdef T8_ENABLE_DEBUG is the preferred variant to write in t8code.

Currently, the usage is split in half:
https://github.com/search?q=repo%3ADLR-AMR%2Ft8code%20%23ifdef%20T8_ENABLE_DEBUG&type=code
https://github.com/search?q=repo%3ADLR-AMR%2Ft8code%20%23if%20T8_ENABLE_DEBUG&type=code

Describe the solution or feature you'd like
Clarify, which variant is preferred and adapt accordingly.

Estimated priority
Which of these is most applicable (remove the others):

"Priority: medium" Should be solved within half a year

[Davknapp]

We decided for #if T8_ENABLE_DEBUG

[cburstedde]

For p4est and sc we use #ifdef: If a feature is not enabled, the macro is indeed not defined at all and #if would throw an error. This seems to be the case with both autoconf and CMake. Wondering why #if seems to be ok with t8code and CMake.