diff --git a/libc/CMakeLists.txt b/libc/CMakeLists.txt
--- a/libc/CMakeLists.txt
+++ b/libc/CMakeLists.txt
@@ -14,6 +14,12 @@
 # The top-level directory in which libc is being built.
 set(LIBC_BUILD_DIR ${CMAKE_CURRENT_BINARY_DIR})
 
+# For a runtimes build we need to manually include tablgen and LLVM directories.
+if ("libc" IN_LIST LLVM_ENABLE_RUNTIMES)
+  include(TableGen)
+  set(LLVM_LIBC_INCLUDE_DIRS ${LLVM_MAIN_INCLUDE_DIR} ${LLVM_BINARY_DIR}/include)
+endif()
+
 # Path libc/scripts directory.
 set(LIBC_BUILD_SCRIPTS_DIR "${LIBC_SOURCE_DIR}/utils/build_scripts")
 
diff --git a/libc/utils/HdrGen/CMakeLists.txt b/libc/utils/HdrGen/CMakeLists.txt
--- a/libc/utils/HdrGen/CMakeLists.txt
+++ b/libc/utils/HdrGen/CMakeLists.txt
@@ -12,7 +12,7 @@
   PublicAPICommand.h
 )
 
-target_include_directories(libc-hdrgen PRIVATE ${LIBC_SOURCE_DIR})
+target_include_directories(libc-hdrgen PRIVATE ${LIBC_SOURCE_DIR} ${LLVM_LIBC_INCLUDE_DIRS})
 target_link_libraries(libc-hdrgen PRIVATE LibcTableGenUtil)
 
 add_subdirectory(PrototypeTestGen)
diff --git a/libc/utils/LibcTableGenUtil/CMakeLists.txt b/libc/utils/LibcTableGenUtil/CMakeLists.txt
--- a/libc/utils/LibcTableGenUtil/CMakeLists.txt
+++ b/libc/utils/LibcTableGenUtil/CMakeLists.txt
@@ -4,4 +4,4 @@
   APIIndexer.h
   LINK_COMPONENTS Support TableGen
 )
-target_include_directories(LibcTableGenUtil PUBLIC ${LIBC_SOURCE_DIR})
+target_include_directories(LibcTableGenUtil PUBLIC ${LIBC_SOURCE_DIR} ${LLVM_LIBC_INCLUDE_DIRS})