refactor: modularize CMakeLists.txt for better maintainability

- Split monolithic 235-line CMakeLists.txt into focused modules
- Main file reduced to 78 lines with clear section organization
- Created 5 specialized modules:
  * ZstdVersion.cmake - CMake policies and version management
  * ZstdOptions.cmake - Build options and platform configuration
  * ZstdDependencies.cmake - External dependency management
  * ZstdBuild.cmake - Build targets and validation
  * ZstdPackage.cmake - Package configuration generation

Benefits:
- Improved readability and maintainability
- Better separation of concerns
- Easier debugging and modification
- Preserved 100% backward compatibility
- All existing build options and targets unchanged

The refactored build system passes all tests and maintains
identical functionality while being much easier to understand
and maintain.

build/cmake/CMakeModules/ZstdDependencies.cmake

@@ -0,0 +1,30 @@
+# ################################################################
+# ZSTD Dependencies Configuration
+# ################################################################
+
+# Function to handle HP-UX thread configuration
+function(setup_hpux_threads)
+    find_package(Threads)
+    if(NOT Threads_FOUND)
+        set(CMAKE_USE_PTHREADS_INIT 1 PARENT_SCOPE)
+        set(CMAKE_THREAD_LIBS_INIT -lpthread PARENT_SCOPE)
+        set(CMAKE_HAVE_THREADS_LIBRARY 1 PARENT_SCOPE)
+        set(Threads_FOUND TRUE PARENT_SCOPE)
+    endif()
+endfunction()
+
+# Configure threading support
+if(ZSTD_MULTITHREAD_SUPPORT AND UNIX)
+    if(CMAKE_SYSTEM_NAME MATCHES "HP-UX")
+        setup_hpux_threads()
+    else()
+        set(THREADS_PREFER_PTHREAD_FLAG ON)
+        find_package(Threads REQUIRED)
+    endif()
+    
+    if(CMAKE_USE_PTHREADS_INIT)
+        set(THREADS_LIBS "${CMAKE_THREAD_LIBS_INIT}")
+    else()
+        message(SEND_ERROR "ZSTD currently does not support thread libraries other than pthreads")
+    endif()
+endif()