build: only add dependencies if needed

The dependencies may be empty which will fail.  Guard against that.

Author: compnerd

cmake/modules/SwiftSupport.cmake

@@ -151,7 +151,9 @@ function(add_swift_target target)
                          ${documentation})
   else()
     add_library(${target}-static STATIC ${objs})
-    add_dependencies(${target}-static ${AST_DEPENDS})
+    if(AST_DEPENDS)
+      add_dependencies(${target}-static ${AST_DEPENDS})
+    endif()
     get_filename_component(ast_output_bn ${AST_OUTPUT} NAME)
     string(REGEX REPLACE "^${CMAKE_STATIC_LIBRARY_PREFIX}" "" ast_output_bn ${ast_output_bn})
     string(REGEX REPLACE "${CMAKE_STATIC_LIBRARY_SUFFIX}$" "" ast_output_bn ${ast_output_bn})