PCRE fails to compile with clang/c++20 and GNU make

PCRE fails to compile:

```
./configure --config=Darwin-clang-c++20
make -s -j16 -C Foundation/
In file included from src/pcre2_tables.c:51:
src/pcre2_internal.h:76:2: error: PCRE2_CODE_UNIT_WIDTH must be defined as 8, 16, or 32.
76 | #error PCRE2_CODE_UNIT_WIDTH must be defined as 8, 16, or 32.
| ^
```
