[libclang] Add dummy libclang-headers target

[smeenai]

Summary

LLVM_DISTRIBUTION_COMPONENTS assumes that each component has both
component and install-component targets. Add a dummy no-op target
for libclang-headers to placate this check.

[smeenai]

Better comment

[akyrtzi]

LGTM.

tools/libclang/CMakeLists.txt

 install(DIRECTORY ../../include/clang-c
   COMPONENT libclang-headers
   DESTINATION "${LIBCLANG_HEADERS_INSTALL_DESTINATION}"
   FILES_MATCHING
   PATTERN "*.h"
   PATTERN ".svn" EXCLUDE
   )
 
+# Add dummy target to placate LLVM_DISTRIBUTION_COMPONENTS.
+add_custom_target(libclang-headers)
+
 if (NOT CMAKE_CONFIGURATION_TYPES) # don't add this for IDE's.
   add_custom_target(install-libclang-headers
     DEPENDS
     COMMAND "${CMAKE_COMMAND}"
             -DCMAKE_INSTALL_COMPONENT=libclang-headers
             -P "${CMAKE_BINARY_DIR}/cmake_install.cmake")
 endif()