diff --git a/libclc/CMakeLists.txt b/libclc/CMakeLists.txt
index 2611818ff902..c98e2043464d 100644
--- a/libclc/CMakeLists.txt
+++ b/libclc/CMakeLists.txt
@@ -72,7 +72,7 @@ else()
   # Note that we check this later (for both build types) but we can provide a
   # more useful error message when built in-tree. We assume that LLVM tools are
   # always available so don't warn here.
-  if( NOT clang IN_LIST LLVM_ENABLE_PROJECTS )
+  if( NOT LLVM_RUNTIMES_BUILD AND NOT clang IN_LIST LLVM_ENABLE_PROJECTS )
     message(FATAL_ERROR "Clang is not enabled, but is required to build libclc in-tree")
   endif()
 
diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
index 1440646762f6..206f009b45f5 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -141,7 +141,7 @@ endforeach()
 # As we migrate runtimes to using the bootstrapping build, the set of default runtimes
 # should grow as we remove those runtimes from LLVM_ENABLE_PROJECTS above.
 set(LLVM_DEFAULT_RUNTIMES "libcxx;libcxxabi;libunwind")
-set(LLVM_SUPPORTED_RUNTIMES "libc;libunwind;libcxxabi;pstl;libcxx;compiler-rt;openmp;llvm-libgcc;offload;flang-rt")
+set(LLVM_SUPPORTED_RUNTIMES "libc;libunwind;libcxxabi;pstl;libcxx;compiler-rt;openmp;llvm-libgcc;offload;flang-rt;libclc")
 set(LLVM_ENABLE_RUNTIMES "" CACHE STRING
   "Semicolon-separated list of runtimes to build, or \"all\" (${LLVM_DEFAULT_RUNTIMES}). Supported runtimes are ${LLVM_SUPPORTED_RUNTIMES}.")
 if(LLVM_ENABLE_RUNTIMES STREQUAL "all")
@@ -204,6 +204,13 @@ if ("flang-rt" IN_LIST LLVM_ENABLE_RUNTIMES)
   endif ()
 endif ()
 
+if ("libclc" IN_LIST LLVM_ENABLE_PROJECTS)
+  message(WARNING "Using LLVM_ENABLE_PROJECTS=libclc is deprecated now, and will "
+    "become a fatal error in the LLVM 21 release.  Please use "
+    "-DLLVM_ENABLE_RUNTIMES=libclc or see the instructions at "
+    "https://libclc.llvm.org/ for building the runtimes.")
+endif()
+
 # Set a shorthand option to enable the GPU build of the 'libc' project.
 option(LIBC_GPU_BUILD "Enable the 'libc' project targeting the GPU" OFF)
 if(LIBC_GPU_BUILD)
diff --git a/llvm/runtimes/CMakeLists.txt b/llvm/runtimes/CMakeLists.txt
index cabadfc9184f..9f86650ec58d 100644
--- a/llvm/runtimes/CMakeLists.txt
+++ b/llvm/runtimes/CMakeLists.txt
@@ -193,7 +193,7 @@ endif()
 
 function(_get_runtime_name name out_var)
   string(FIND ${name} "lib" idx)
-  if(idx EQUAL 0 AND NOT ${name} STREQUAL "libc")
+  if(idx EQUAL 0 AND NOT (${name} STREQUAL "libc" OR ${name} STREQUAL "libclc"))
     string(SUBSTRING ${name} 3 -1 name)
   endif()
   set(${out_var} ${name} PARENT_SCOPE)
diff --git a/runtimes/CMakeLists.txt b/runtimes/CMakeLists.txt
index 7f1e2ae065d6..878b2eee3861 100644
--- a/runtimes/CMakeLists.txt
+++ b/runtimes/CMakeLists.txt
@@ -35,7 +35,7 @@ list(INSERT CMAKE_MODULE_PATH 0
 
 # We order libraries to mirror roughly how they are layered, except that compiler-rt can depend
 # on libc++, so we put it after.
-set(LLVM_DEFAULT_RUNTIMES "libc;libunwind;libcxxabi;pstl;libcxx;compiler-rt;openmp;offload")
+set(LLVM_DEFAULT_RUNTIMES "libc;libunwind;libcxxabi;pstl;libcxx;compiler-rt;libclc;openmp;offload")
 set(LLVM_SUPPORTED_RUNTIMES "${LLVM_DEFAULT_RUNTIMES};llvm-libgcc;flang-rt")
 set(LLVM_ENABLE_RUNTIMES "" CACHE STRING
   "Semicolon-separated list of runtimes to build, or \"all\" (${LLVM_DEFAULT_RUNTIMES}). Supported runtimes are ${LLVM_SUPPORTED_RUNTIMES}.")