fix(ApacheConnector): build as C++20 with clang; compare the scheme by content in secure() #5501

[matejk]

Summary

Fixes #5501. Two fixes in ApacheConnector/src/ApacheConnector.cpp, one commit each.

• C++20 build with clang. The definition module AP_MODULE_DECLARE_DATA poco_module = ... starts a line with module followed by a name, which C++20 treats as a module directive; clang 21 rejects it. The definition now starts with extern "C", like the forward declaration near the top of the file and the other entry points. The linkage does not change (the forward declaration already gave the variable C linkage), and mod_poco.so still exports poco_module as a C data symbol.
  AP_DECLARE_MODULE(poco) from http_config.h would also avoid the directive, but it adds APLOG_USE_MODULE, which changes the module index that log calls carry, so it is not used here.
• ApacheRequestRec::secure(). It compared ap_http_scheme() (a const char*) with "https" by address, so it returned false for HTTPS requests. It now compares the text and handles a null scheme. clang flagged this with -Wstring-compare in the same build.

Verification

Ubuntu 26.04, clang 21.1.8, apache2-dev 2.4.66, -DPOCO_MINIMAL_BUILD=ON -DENABLE_APACHECONNECTOR=ON:

• main: mod_poco fails as C++20 with the three module errors from ApacheConnector: mod_poco does not compile as C++20 with clang; secure() compares the scheme by address #5501.
• This branch: mod_poco builds as C++20 without warnings; ApacheConnector.cpp also compiles as C++17. nm shows D poco_module in the C++17 object before and after the change and in libmod_poco.so.124.
• clang-tidy reports nothing on the changed lines.

CI builds ApacheConnector only in the GCC sanitizer jobs, which accept the old line, so it does not cover the clang C++20 case.