cmake: add C++ integration test

Also:
- include code to verify a C++-specific public header regression
  reported in 8.19.0-rc2.
- curl/curl.h: mention C++ global namespace in comment.
- GHA/dist: add CI job for C++. Runtime: 15 seconds.

Follow-up to ee9b000438 #20686
Ref: #20682

Closes #20687

Makefile.am

@@ -62,6 +62,7 @@ CMAKE_DIST =                                     \
   CMakeLists.txt                                 \
   tests/cmake/CMakeLists.txt                     \
   tests/cmake/test.c                             \
+  tests/cmake/test.cpp                           \
   tests/cmake/test.sh
 
 EXTRA_DIST = CHANGES.md COPYING RELEASE-NOTES Dockerfile .clang-tidy.yml .editorconfig $(CMAKE_DIST)