Attempt to fix ZLIB CMake logic on Windows

CMake doesn't seem to like it when you regex search for "^".

(cherry picked from commit 1079c68)

Author: rnk

llvm/lib/Support/CMakeLists.txt

@@ -198,10 +198,12 @@ if(LLVM_ENABLE_ZLIB)
     get_property(zlib_library TARGET ZLIB::ZLIB PROPERTY LOCATION)
   endif()
   get_filename_component(zlib_library ${zlib_library} NAME)
-  if(zlib_library MATCHES "^${CMAKE_STATIC_LIBRARY_PREFIX}.*${CMAKE_STATIC_LIBRARY_SUFFIX}$")
+  if(CMAKE_STATIC_LIBRARY_PREFIX AND
+     zlib_library MATCHES "^${CMAKE_STATIC_LIBRARY_PREFIX}.*${CMAKE_STATIC_LIBRARY_SUFFIX}$")
     STRING(REGEX REPLACE "^${CMAKE_STATIC_LIBRARY_PREFIX}" "" zlib_library ${zlib_library})
     STRING(REGEX REPLACE "${CMAKE_STATIC_LIBRARY_SUFFIX}$" "" zlib_library ${zlib_library})
-  elseif(zlib_library MATCHES "^${CMAKE_SHARED_LIBRARY_PREFIX}.*${CMAKE_SHARED_LIBRARY_SUFFIX}$")
+  elseif(CMAKE_SHARED_LIBRARY_PREFIX AND
+         zlib_library MATCHES "^${CMAKE_SHARED_LIBRARY_PREFIX}.*${CMAKE_SHARED_LIBRARY_SUFFIX}$")
     STRING(REGEX REPLACE "^${CMAKE_SHARED_LIBRARY_PREFIX}" "" zlib_library ${zlib_library})
     STRING(REGEX REPLACE "${CMAKE_SHARED_LIBRARY_SUFFIX}$" "" zlib_library ${zlib_library})
   endif()