[Polly] Fix the build for non-`MSVC` Windows targets.

[DiamondLovesYou]

Summary

Rely on LLVM's LLVM_ENABLE_PLUGINS cmake variable for deciding
when to build the loadable modules.

[grosser]

This is a good idea.

[DiamondLovesYou]

See D51963.

cmake/CMakeLists.txt

 # Keep this in sync with llvm/cmake/CMakeLists.txt!
 
 set(LLVM_INSTALL_PACKAGE_DIR "lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm")
 set(POLLY_INSTALL_PACKAGE_DIR "lib${LLVM_LIBDIR_SUFFIX}/cmake/polly")
 if (CMAKE_CONFIGURATION_TYPES)
   set(POLLY_EXPORTS_FILE_NAME "PollyExports-$<LOWER_CASE:$<CONFIG>>.cmake")
 else()
   # avoid conflicts in the build-tree when changing configuration
   set(POLLY_EXPORTS_FILE_NAME "PollyExports-all.cmake")
 endif()
 
 set(POLLY_CONFIG_EXPORTED_TARGETS Polly ${ISL_TARGET})
-if (NOT MSVC)
+if (LLVM_ENABLE_PLUGINS)
   # LLVMPolly is a dummy target on Win
   list(APPEND POLLY_CONFIG_EXPORTED_TARGETS LLVMPolly)
 endif()
 if (POLLY_ENABLE_GPGPU_CODEGEN)
   list(APPEND POLLY_CONFIG_EXPORTED_TARGETS PollyPPCG)
 endif()
 
 # Get the target type for every exported target

lib/CMakeLists.txt

target_link_libraries(Polly PUBLIC
     LLVMProfileData
     LLVMTarget
     LLVMVectorize
     )
 endif ()
 
 # Create a loadable module Polly.so that can be loaded using
 # LLVM's/clang's "-load" option.
-if (MSVC)
+if (NOT LLVM_ENABLE_PLUGINS)
   # Add dummy target, because loadable modules are not supported on Windows
   add_custom_target(LLVMPolly)
   set_target_properties(LLVMPolly PROPERTIES FOLDER "Polly")
 else ()
   add_polly_loadable_module(LLVMPolly
     Polly.cpp
     $<TARGET_OBJECTS:PollyCore>
   )