[gn] port 40cc437

Author: nico

llvm/utils/gn/secondary/clang/test/BUILD.gn

@@ -48,6 +48,7 @@ write_lit_config("lit_site_cfg") {
     "LLVM_LINK_LLVM_DYLIB=0",
     "CLANG_LINK_CLANG_DYLIB=0",
     "LLVM_WITH_Z3=0",
+    "Z3_INCLUDE_DIR=",
     "CLANG_BUILD_EXAMPLES=0",
     "CLANG_BUILT_STANDALONE=0",
     "CLANG_DEFAULT_CXX_STDLIB=",  # Empty string means "default value" here.