fix(CMake): correct target variable for DLLs

The `_dll` variable contains the target name. The code
was trying to use `_NAME` when handling ABSL_PROPAGATE_CXX_STD.
Probably a cut&paste error from the other places where we
handle that option.

PiperOrigin-RevId: 510551097
Change-Id: I006435978333dd7a72a300503389a7bf48fb6d0d

CMake/AbseilDll.cmake

@@ -789,7 +789,7 @@ Cflags: -I\${includedir}${PC_CFLAGS}\n")
     # Abseil libraries require C++14 as the current minimum standard. When
     # compiled with C++17 (either because it is the compiler's default or
     # explicitly requested), then Abseil requires C++17.
-    _absl_target_compile_features_if_available(${_NAME} PUBLIC ${ABSL_INTERNAL_CXX_STD_FEATURE})
+    _absl_target_compile_features_if_available(${_dll} PUBLIC ${ABSL_INTERNAL_CXX_STD_FEATURE})
   else()
     # Note: This is legacy (before CMake 3.8) behavior. Setting the
     # target-level CXX_STANDARD property to ABSL_CXX_STANDARD (which is
@@ -799,8 +799,8 @@ Cflags: -I\${includedir}${PC_CFLAGS}\n")
     # CXX_STANDARD_REQUIRED does guard against the top-level CMake project
     # not having enabled CMAKE_CXX_STANDARD_REQUIRED (which prevents
     # "decaying" to an older standard if the requested one isn't available).
-    set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD ${ABSL_CXX_STANDARD})
-    set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD_REQUIRED ON)
+    set_property(TARGET ${_dll} PROPERTY CXX_STANDARD ${ABSL_CXX_STANDARD})
+    set_property(TARGET ${_dll} PROPERTY CXX_STANDARD_REQUIRED ON)
   endif()
 
   install(TARGETS ${_dll} EXPORT ${PROJECT_NAME}Targets