[ASTMatchers] Fix build when no targets are enabled

This makes sense to do when building only tools like clang-tidy for
example.

Differential Revision: https://reviews.llvm.org/D93987

diff --git a/llvm/cmake/modules/LLVM-Config.cmake b/llvm/cmake/modules/LLVM-Config.cmake
index ebe13e7..5d9ec79 100644 (file)
--- a/llvm/cmake/modules/LLVM-Config.cmake
+++ b/llvm/cmake/modules/LLVM-Config.cmake
@@ -28,7 +28,9 @@ function(is_llvm_target_library library return_var)
     string(TOUPPER "${LLVM_TARGETS_TO_BUILD}" targets)
   elseif(ARG_OMITTED_TARGETS)
     set(omitted_targets ${LLVM_ALL_TARGETS})
-    list(REMOVE_ITEM omitted_targets ${LLVM_TARGETS_TO_BUILD})
+    if (LLVM_TARGETS_TO_BUILD)
+      list(REMOVE_ITEM omitted_targets ${LLVM_TARGETS_TO_BUILD})
+    endif()
     string(TOUPPER "${omitted_targets}" targets)
   else()
     string(TOUPPER "${LLVM_ALL_TARGETS}" targets)