Revert upstream PR #3023 ("Bump LibreSSL and use it for libcvmfs_crypto")
for Gentoo and unbundle libressl.

libcvmfs_crypto links against the system OpenSSL again.  The symbol
isolation the PR was aiming for is preserved: cvmfs_crypto keeps its
existing -fvisibility=hidden and -Wl,--exclude-libs,ALL flags (used with
the bundled static LibreSSL), so it exports only the cvmfs C++ API and no
libcrypto symbols, and at runtime it simply DT_NEEDEDs the same
libcrypto.so.3 that curl/cvmfs2 already use -- the same situation as
before the PR.

diff --git a/CMakeLists.txt b/CMakeLists.txt
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -427,6 +427,9 @@ endif ()
 find_package (OpenSSL REQUIRED)
 set (INCLUDE_DIRECTORIES ${INCLUDE_DIRECTORIES} ${OPENSSL_INCLUDE_DIR})
 
+set(Libcrypto_LIBRARIES OpenSSL::Crypto)
+set(Libcrypto_INCLUDE_DIRS ${OPENSSL_INCLUDE_DIR})
+
 find_package (Libcrypto REQUIRED)
 # We do not add the Libcrypto include directories to INCLUDE_DIRECTORIES
 # to avoid a clash with the system openssl. We only use it for libcvmfs_crypto
diff --git a/cvmfs/crypto/openssl_version.h b/cvmfs/crypto/openssl_version.h
--- a/cvmfs/crypto/openssl_version.h
+++ b/cvmfs/crypto/openssl_version.h
@@ -9,12 +9,6 @@
 
 // Safeguard when compiling libcvmfs_crypto: make sure we pick up the built-in
 // LibreSSL and not the system's OpenSSL
-#ifdef CVMFS_LIBRARY
-#ifndef LIBRESSL_VERSION_NUMBER
-#error "picking up OpenSSL includes instead of LibreSSL"
-#endif
-#endif
-
 #if OPENSSL_VERSION_NUMBER >= 0x10100000L
 #define OPENSSL_API_INTERFACE_V11
 #if OPENSSL_VERSION_NUMBER >= 0x10101000L