diff --git a/libc/CMakeLists.txt b/libc/CMakeLists.txt
--- a/libc/CMakeLists.txt
+++ b/libc/CMakeLists.txt
@@ -28,7 +28,9 @@
   endif()
 endif()
 
-if("libc" IN_LIST LLVM_ENABLE_RUNTIMES AND NOT LLVM_RUNTIMES_BUILD)
+option(LIBC_HDRGEN_ONLY "Only build the 'libc-hdrgen' executable" OFF)
+if(("libc" IN_LIST LLVM_ENABLE_RUNTIMES AND NOT LLVM_RUNTIMES_BUILD) OR 
+   LIBC_HDRGEN_ONLY)
   # When libc is build as part of the runtimes/bootstrap build's CMake run, we
   # only need to build the host tools to build the libc. So, we just do enough
   # to build libc-hdrgen and return.