refactor: replace vendored xxhash.h with CMake FetchContent dependency

Remove 7490-line vendored xxhash.h, use FetchContent to pull xxHash v0.8.3
at CMake configure time (SOURCE_SUBDIR cmake_unofficial). Links as static
library via the xxhash target.

CMakeLists.txt

@@ -11,6 +11,15 @@ add_compile_options(-Wall -g -O3)
 
 # add_link_options(-fsanitize=address)
 
+include(FetchContent)
+FetchContent_Declare(
+  xxhash
+  GIT_REPOSITORY https://github.com/Cyan4973/xxHash
+  GIT_TAG v0.8.3
+  SOURCE_SUBDIR cmake_unofficial
+)
+FetchContent_MakeAvailable(xxhash)
+
 set(THREADS_PREFER_PTHREAD_FLAG ON)
 find_package(Threads REQUIRED)
 
@@ -28,13 +37,13 @@ file(GLOB TEST_SRCS "tests/*.c")
 
 add_executable(server ${SERVER_SRCS} ${SHARED_SRCS})
 target_include_directories(server PRIVATE src/shared src/server src/client)
-target_link_libraries(server PRIVATE Threads::Threads ${ZSTD_LIBRARY} OpenSSL::SSL OpenSSL::Crypto)
+target_link_libraries(server PRIVATE Threads::Threads ${ZSTD_LIBRARY} OpenSSL::SSL OpenSSL::Crypto xxhash)
 
 add_executable(client ${CLIENT_SRCS} ${SHARED_SRCS})
 target_include_directories(client PRIVATE src/shared src/server src/client)
-target_link_libraries(client PRIVATE Threads::Threads ${ZSTD_LIBRARY} OpenSSL::SSL OpenSSL::Crypto)
+target_link_libraries(client PRIVATE Threads::Threads ${ZSTD_LIBRARY} OpenSSL::SSL OpenSSL::Crypto xxhash)
 
 add_executable(tests ${TEST_SRCS} ${SHARED_SRCS} src/client/scanner.c)
 target_include_directories(tests PRIVATE tests src/shared src/server src/client)
-target_link_libraries(tests PRIVATE Threads::Threads ${ZSTD_LIBRARY} OpenSSL::SSL OpenSSL::Crypto)
+target_link_libraries(tests PRIVATE Threads::Threads ${ZSTD_LIBRARY} OpenSSL::SSL OpenSSL::Crypto xxhash)
 

src/shared/delta.c

@@ -5,7 +5,7 @@
 
 #define XXH_STATIC_LINKING_ONLY
 #define XXH_IMPLEMENTATION
-#include "xxhash.h"
+#include <xxhash.h>
 
 uint32_t delta_adler32(const void *data, uint32_t len) {
   const uint8_t *p = (const uint8_t *)data;