build: fix macOS link options + libtor paths for all 7 CI jobs

Run #467 (the re-trigger after #466's fixes) failed with two new error
classes that the previous commit didn't catch:

  1. macOS link error:
     ld: unknown options: --allow-multiple-definition --start-group --end-group
     src/CMakeLists.txt passed GNU ld flags unconditionally in the
     USE_TOR_EMBEDDED block. Apple's ld64 doesn't recognize them.
     Guard the GNU-only options with NOT APPLE; keep -ltor and the
     linkable libraries outside the guard so macOS still gets them.

  2. Linux libtor configure error:
     configure: error: "You must specify an explicit
     --with-libevent-dir=x option when using --enable-static-libevent"
     build-libtor.sh defaults to /mingw64 paths. On ubuntu-22.04 the
     libevent-dev/libssl-dev/zlib1g-dev packages install under /usr,
     so the libevent flag was being silently dropped. Set
     LIBEVENT_DIR=/usr OPENSSL_DIR=/usr ZLIB_DIR=/usr for Linux jobs.

  3. Added the build-libtor step to three more jobs that needed it
     (Qt GUI builds also link -ltor transitively via triangles_common):
       - build-windows-qt
       - build-linux-qt
       - build-macos

After this:
  - All 7 build jobs will pass the libtor step.
  - macOS Qt link will work (no more GNU-ld-only options).
  - Windows Qt build will produce the .exe installer artifact.

If anything still fails I'll iterate. This is the third build pass.

src/CMakeLists.txt

@@ -179,14 +179,24 @@ if(USE_TOR_EMBEDDED)
     # and its dependencies.
     # Use --allow-multiple-definition because libtor.a may pull in static
     # OpenSSL objects that duplicate the DLL import lib already linked above.
+    # These GNU ld options are not supported on macOS (which uses lld) —
+    # guard with NOT APPLE so the build still works on macOS.
+    if(NOT APPLE)
+        target_link_libraries(triangles_common PUBLIC
+            -Wl,--allow-multiple-definition
+            -Wl,--start-group
+        )
+    endif()
     target_link_libraries(triangles_common PUBLIC
-        -Wl,--allow-multiple-definition
-        -Wl,--start-group
         -ltor
         -levent -levent_core -levent_extra -levent_openssl
         -lssl -lcrypto -lz -llzma -lzstd
-        -Wl,--end-group
     )
+    if(NOT APPLE)
+        target_link_libraries(triangles_common PUBLIC
+            -Wl,--end-group
+        )
+    endif()
     if(WIN32)
         target_link_libraries(triangles_common PUBLIC iphlpapi shlwapi crypt32)
     endif()