[Build/CI] Set FETCHCONTENT_BASE_DIR to one location for better caching (#8930)

.gitignore

@@ -33,6 +33,7 @@ share/python-wheels/
 .installed.cfg
 *.egg
 MANIFEST
+/.deps/
 
 # PyInstaller
 #  Usually these files are written by a python script from a template

CMakeLists.txt

@@ -166,7 +166,16 @@ if(NVCC_THREADS AND VLLM_GPU_LANG STREQUAL "CUDA")
   list(APPEND VLLM_GPU_FLAGS "--threads=${NVCC_THREADS}")
 endif()
 
+
+#
+# Use FetchContent for C++ dependencies that are compiled as part of vLLM's build process.
+# Configure it to place files in vllm/.deps, in order to play nicely with sccache.
+#
 include(FetchContent)
+get_filename_component(PROJECT_ROOT_DIR "${CMAKE_CURRENT_SOURCE_DIR}" ABSOLUTE)
+file(MAKE_DIRECTORY "${FETCHCONTENT_BASE_DIR}")
+set(FETCHCONTENT_BASE_DIR "${PROJECT_ROOT_DIR}/.deps")
+message(STATUS "FetchContent base directory: ${FETCHCONTENT_BASE_DIR}")
 
 #
 # Define other extension targets