cmake: link CMAKE_DL_LIBS into the static findopensslfeatures probe

[jolavillette]

Follow-up to #2396 (cross-compilation with the OpenSSL backend).

When CMAKE_CROSSCOMPILING_EMULATOR is set the findopensslfeatures probe is linked with -static. A static OpenSSL 1.1.1 libcrypto.a pulls in dso_dlfcn.o, which references dlopen/dlsym/dlclose/dlerror, and CMake's FindOpenSSL only appends CMAKE_DL_LIBS to OpenSSL::Crypto when pkg-config lists it in Libs.private — which a bare cross sysroot does not have. The probe then fails to link:

ld.lld: error: undefined symbol: dlopen
>>> referenced by dso_dlfcn.c
>>>               dso_dlfcn.o:(dlfcn_load) in archive .../sysroot/usr/lib/libcrypto.a
...
CMake Error at cmake/Modules/FindOpenSSLFeatures.cmake:149 (message):
  Error building findopensslfeatures

This links ${CMAKE_DL_LIBS} explicitly in the same CMAKE_CROSSCOMPILING_EMULATOR branch that adds -static. It is added as a library rather than a link option so it is ordered after libcrypto.a for linkers that resolve archives in a single pass; where CMAKE_DL_LIBS is empty it is a no-op (target_link_libraries(t PRIVATE ) is valid).

Tested

Android NDK r29 (aarch64-linux-android24-clang, ld.lld), OpenSSL 1.1.1n static, CRYPTO_BACKEND=openssl, CMAKE_CROSSCOMPILING_EMULATOR=/usr/bin/qemu-aarch64, host CMake 3.28.3 — the RetroShare Android toolchain (RetroShare/libretroshare#376):

• main (ee97ed7): the four undefined symbol: dl* errors above, Error building findopensslfeatures.
• this branch: configures, the probe runs under qemu and enumerates the features, librnp.a builds and installs.

Native (non-cross) builds do not enter the branch and are unchanged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.40%. Comparing base (ee97ed7) to head (2fb949a).
⚠️ Report is 32 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2473   +/-   ##
=======================================
  Coverage   85.40%   85.40%           
=======================================
  Files         126      126           
  Lines       22966    22966           
=======================================
  Hits        19614    19614           
  Misses       3352     3352           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[ronaldtse]

Thank you @jolavillette — this is a clean, well-scoped fix for a real gap in the #2396 cross-compile support. The explanation is spot-on: -static pulls in dso_dlfcn.o, bare sysroots lack pkg-config Libs.private, and CMAKE_DL_LIBS never gets added. Linking it explicitly in the same branch that adds -static is exactly right, and the library-vs-flag ordering ensures single-pass linkers resolve it after the archive. Confirmed this matches the failure mode we saw on the OHOS side. Approving.

[ronaldtse]

@ni4 this needs your approval (1 of 2): #2473

One-line fix by @jolavillette for a real cross-compile gap from #2396: the findopensslfeatures probe builds with -static under CMAKE_CROSSCOMPILING_EMULATOR, and static libcrypto.a references dlopen/dlsym that bare cross sysroots don't link. Adding CMAKE_DL_LIBS explicitly in the same branch. Tested against Android NDK + qemu — fails on main, passes with this.