Patch to unbundle dependencies to build in Gentoo.

 - SHA3 -> externals/sha3 shipped in the tarball, built as a static library
   with the same sources as the built-in externals build.
 - JSON -> the system dev-cpp/nlohmann_json (the externals/json machinery is
   only used with BUILTIN_EXTERNALS; without it no nlohmann_json target
   would exist).  nlohmann_json is header-only but its C++ headers need the
   C++ standard library, hence the C project type.
 - Protobuf -> link the imported protobuf::libprotobuf-lite target instead of
   the bare ${PROTOBUF_LITE_LIBRARY} path.  The imported target propagates
   compile features from its dependencies (including absl), without
   hardcoding a C++ standard in the ebuild.

diff --git a/CMakeLists.txt b/CMakeLists.txt
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -443,7 +443,7 @@ if (BUILD_CVMFS OR BUILD_LIBCVMFS OR BUILD_SERVER OR BUILD_SERVER_DEBUG OR
   find_package (ZLIB REQUIRED)
   set (INCLUDE_DIRECTORIES ${INCLUDE_DIRECTORIES} ${ZLIB_INCLUDE_DIRS})
 
-  find_package (SHA3 REQUIRED)
+  add_subdirectory(externals/sha3)
   set (INCLUDE_DIRECTORIES ${INCLUDE_DIRECTORIES} ${SHA3_INCLUDE_DIRS})
 endif ()
 
diff --git a/CMakeLists.txt b/CMakeLists.txt
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -455,0 +457,2 @@ if (BUILD_CVMFS OR BUILD_LIBCVMFS OR BUILD_SERVER OR BUILD_SERVER_DEBUG OR
+
+  find_package (nlohmann_json REQUIRED)
+
diff --git a/CMakeLists.txt b/CMakeLists.txt
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -498,5 +498,7 @@ if (BUILD_CVMFS OR BUILD_LIBCVMFS OR BUILD_LIBCVMFS_CACHE OR
     BUILD_UNITTESTS OR BUILD_UNITTESTS_DEBUG OR BUILD_UBENCHMARKS OR
     BUILD_SHRINKWRAP)
   find_package(Protobuf REQUIRED)
+  find_package(protobuf CONFIG REQUIRED)
+  set(PROTOBUF_LITE_LIBRARY protobuf::libprotobuf-lite)
   set (INCLUDE_DIRECTORIES ${INCLUDE_DIRECTORIES} ${PROTOBUF_INCLUDE_DIRS})
 endif ()
diff --git a/externals/sha3/CMakeLists.txt b/externals/sha3/CMakeLists.txt
new file mode 100644
--- /dev/null
+++ b/externals/sha3/CMakeLists.txt
@@ -0,0 +1,21 @@
+project(SHA3 C CXX)
+
+# Gentoo: build the bundled Keccak/SHA3 code as a static library instead of
+# requiring a system SHA3 package (no such package exists in Gentoo).
+add_library(cvmfs_sha3 STATIC
+  src/64opt/KeccakF-1600-opt64.c
+  src/KeccakHash.c
+  src/KeccakSponge.c
+  src/SnP-FBWL-default.c
+)
+
+set_property(TARGET cvmfs_sha3 PROPERTY POSITION_INDEPENDENT_CODE TRUE)
+
+target_include_directories(cvmfs_sha3 PRIVATE
+  ${CMAKE_CURRENT_SOURCE_DIR}/src
+  ${CMAKE_CURRENT_SOURCE_DIR}/src/64opt
+)
+
+set(SHA3_LIBRARY $<TARGET_FILE:cvmfs_sha3> PARENT_SCOPE)
+set(SHA3_LIBRARIES cvmfs_sha3 PARENT_SCOPE)
+set(SHA3_INCLUDE_DIRS ${CMAKE_CURRENT_SOURCE_DIR}/src ${CMAKE_CURRENT_SOURCE_DIR}/src/64opt PARENT_SCOPE)
diff --git a/CMakeLists.txt b/CMakeLists.txt
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -455,0 +457,2 @@ if (BUILD_CVMFS OR BUILD_LIBCVMFS OR BUILD_SERVER OR BUILD_SERVER_DEBUG OR
+
+  find_package (nlohmann_json REQUIRED)