[openssl] builtin: destination of include files was not correct

[ferdymercury]

Compilation on mac of XRootd and/or civetweb with builtin_openssl probably just worked fine because it picked system-wide openssl headers rather than the builtin ones.

Found out while investigating failures in #21947 of the type
openssl/bn.h not found even if it was correctly doing `include_directories("/Users/sftnight/ROOT-CI/build/builtins/openssl/OPENSSL-prefix/include")
See https://github.com/root-project/root/actions/runs/24824049926/job/72655476462

2026-04-23T08:10:32.7957500Z -- Found OpenSSL: /Users/sftnight/ROOT-CI/build/builtins/openssl/OPENSSL-prefix/lib/libcrypto.a
2026-04-23T08:10:32.7957990Z -- OpenSSL include directory: /Users/sftnight/ROOT-CI/build/builtins/openssl/OPENSSL-prefix/include
2026-04-23T08:10:32.7958410Z CMake Warning at src/CMakeLists.txt:60 (message):
2026-04-23T08:10:32.7958700Z 
2026-04-23T08:10:32.7958740Z  ---> HERE glob returns no headers .h at all in that folder or any subdir

/Users/sftnight/ROOT-CI/build/builtins/CIVETWEB-prefix/src/BUILTIN_CIVETWEB/src/civetweb.c:1751:10: fatal error: 'openssl/bn.h' file not found
2026-04-23T08:10:32.9740120Z  1751 | #include <openssl/bn.h>
2026-04-23T08:10:32.9740590Z       |          ^~~~~~~~~~~~~~

Note that Makefile.in of the openssl repository has extra logic depending on if openssldir is passed or not, and modifies the destination suffixes accordingly.

[github-actions]

Test Results

    22 files      22 suites   3d 8h 51m 53s ⏱️
 3 849 tests  3 847 ✅  1 💤 1 ❌
76 862 runs  76 843 ✅ 18 💤 1 ❌

For more details on these failures, see this check.

Results for commit e3e1e6d.

♻️ This comment has been updated with latest results.

[dpiparo]

/backport to 6.40

[root-project-bot]

Preparing to backport PR #22026 to branch 6.40 requested by dpiparo

[root-project-bot]

This PR has been backported to branch 6.40: #22074