ci: Specify -Wno-overlength-strings on macOS

This disables the following warning, which was causing CI failures on
macOS when building the libpcre2 subproject:
```
../subprojects/pcre2-10.40/src/pcre2_error.c:66:3: error: string literal of length 4380 exceeds maximum length 4095 that ISO C99 compilers are required to support [-Werror,-Woverlength-strings]
```

We don’t want to explicitly rely on using overlength strings in GLib,
which is why this change is a `CFLAGS` in the CI configuration, rather
than setting a project-level argument in `meson.build`.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

.gitlab-ci.yml

@@ -437,6 +437,10 @@ macos:
   needs: []
   variables:
     GIT_SUBMODULE_STRATEGY: normal
+    # libpcre2 gets built as a subproject, but the default Clang options on
+    # macOS limit string lengths to 4095B — pcre2_error.c has a string of length
+    # 4380B
+    CFLAGS: '-Wno-overlength-strings'
   before_script:
     - bash .gitlab-ci/show-execution-environment.sh
     - pip3 install --user meson==0.60.1