This issue is already known to upstream but a new release with a fix is still pending: https://github.com/libgit2/libgit2/issues/4220

Here is how the build fails:

/usr/bin/ld: warning: libssl.so.8, needed by /usr/local/lib/libssh2.so, may conflict with libssl.so.43
//usr/lib/libssl.so.8: warning: warning: EVP_DecryptFinal is often misused, please use EVP_DecryptFinal_ex and EVP_CIPHER_CTX_cleanup
//usr/lib/libssl.so.8: warning: warning: EVP_EncryptFinal is often misused, please use EVP_EncryptFinal_ex and EVP_CIPHER_CTX_cleanup
/usr/bin/ld: warning: libcrypto.so.8, needed by /usr/local/lib/libssh2.so, may conflict with libcrypto.so.41
//lib/libcrypto.so.8: warning: warning: EVP_CipherFinal is often misused, please use EVP_CipherFinal_ex and EVP_CIPHER_CTX_cleanup
CMakeFiles/libgit2_clar.dir/src/openssl_stream.c.o: In function `git_openssl_stream_global_init':
/usr/ports/devel/libgit2/work/libgit2-0.25.1/src/openssl_stream.c:(.text+0xa0): undefined reference to `BIO_get_new_index'
/usr/ports/devel/libgit2/work/libgit2-0.25.1/src/openssl_stream.c:(.text+0xb8): undefined reference to `BIO_meth_new'
/usr/ports/devel/libgit2/work/libgit2-0.25.1/src/openssl_stream.c:(.text+0xd8): undefined reference to `BIO_meth_set_write'
/usr/ports/devel/libgit2/work/libgit2-0.25.1/src/openssl_stream.c:(.text+0xed): undefined reference to `BIO_meth_set_read'
/usr/ports/devel/libgit2/work/libgit2-0.25.1/src/openssl_stream.c:(.text+0x102): undefined reference to `BIO_meth_set_puts'
/usr/ports/devel/libgit2/work/libgit2-0.25.1/src/openssl_stream.c:(.text+0x117): undefined reference to `BIO_meth_set_gets'
/usr/ports/devel/libgit2/work/libgit2-0.25.1/src/openssl_stream.c:(.text+0x12c): undefined reference to `BIO_meth_set_ctrl'
/usr/ports/devel/libgit2/work/libgit2-0.25.1/src/openssl_stream.c:(.text+0x141): undefined reference to `BIO_meth_set_create'
/usr/ports/devel/libgit2/work/libgit2-0.25.1/src/openssl_stream.c:(.text+0x156): undefined reference to `BIO_meth_set_destroy'
CMakeFiles/libgit2_clar.dir/src/openssl_stream.c.o: In function `shutdown_ssl':
/usr/ports/devel/libgit2/work/libgit2-0.25.1/src/openssl_stream.c:(.text+0x1a3): undefined reference to `BIO_meth_free'
CMakeFiles/libgit2_clar.dir/src/openssl_stream.c.o: In function `openssl_connect':
/usr/ports/devel/libgit2/work/libgit2-0.25.1/src/openssl_stream.c:(.text+0x35e): undefined reference to `BIO_set_data'
/usr/ports/devel/libgit2/work/libgit2-0.25.1/src/openssl_stream.c:(.text+0x495): undefined reference to `ASN1_STRING_get0_data'
/usr/ports/devel/libgit2/work/libgit2-0.25.1/src/openssl_stream.c:(.text+0x63d): undefined reference to `ASN1_STRING_get0_data'
CMakeFiles/libgit2_clar.dir/src/openssl_stream.c.o: In function `bio_write':
/usr/ports/devel/libgit2/work/libgit2-0.25.1/src/openssl_stream.c:(.text+0xb2f): undefined reference to `BIO_get_data'
CMakeFiles/libgit2_clar.dir/src/openssl_stream.c.o: In function `bio_read':
/usr/ports/devel/libgit2/work/libgit2-0.25.1/src/openssl_stream.c:(.text+0xb5f): undefined reference to `BIO_get_data'
CMakeFiles/libgit2_clar.dir/src/openssl_stream.c.o: In function `bio_puts':
/usr/ports/devel/libgit2/work/libgit2-0.25.1/src/openssl_stream.c:(.text+0xba1): undefined reference to `BIO_get_data'
CMakeFiles/libgit2_clar.dir/src/openssl_stream.c.o: In function `bio_create':
/usr/ports/devel/libgit2/work/libgit2-0.25.1/src/openssl_stream.c:(.text+0xbf1): undefined reference to `BIO_set_init'
/usr/ports/devel/libgit2/work/libgit2-0.25.1/src/openssl_stream.c:(.text+0xbfd): undefined reference to `BIO_set_data'
CMakeFiles/libgit2_clar.dir/src/openssl_stream.c.o: In function `bio_destroy':
/usr/ports/devel/libgit2/work/libgit2-0.25.1/src/openssl_stream.c:(.text+0xc20): undefined reference to `BIO_set_data'
cc: error: linker command failed with exit code 1 (use -v to see invocation)
*** [libgit2_clar] Error code 1

make[4]: stopped in /usr/ports/devel/libgit2/work/libgit2-0.25.1
1 error

make[4]: stopped in /usr/ports/devel/libgit2/work/libgit2-0.25.1
*** [CMakeFiles/libgit2_clar.dir/all] Error code 2

make[3]: stopped in /usr/ports/devel/libgit2/work/libgit2-0.25.1
1 error

make[3]: stopped in /usr/ports/devel/libgit2/work/libgit2-0.25.1
*** [all] Error code 2

make[2]: stopped in /usr/ports/devel/libgit2/work/libgit2-0.25.1
1 error

make[2]: stopped in /usr/ports/devel/libgit2/work/libgit2-0.25.1
===> Compilation failed unexpectedly.
Try to set MAKE_JOBS_UNSAFE=yes and rebuild before reporting the failure to
the maintainer.
*** Error code 1

Stop.
make[1]: stopped in /usr/ports/devel/libgit2
*** Error code 1

Stop.
make: stopped in /usr/ports/devel/libgit2

===>>> make build failed for devel/libgit2
===>>> Aborting update

===>>> Update for devel/libgit2 failed
===>>> Aborting update