[lldb/test] Fix TestCppIncompleteTypeMembers.py

modify the Makefile.rules line which was interfering with the
target-specific variable values.

lldb/packages/Python/lldbsuite/test/make/Makefile.rules

@@ -367,7 +367,7 @@ ifeq "$(OS)" "Windows_NT"
 		# MSVC 2015 or higher is required, which depends on c++14, so
 		# append these values unconditionally.
 		CXXFLAGS += -fms-compatibility-version=19.0
-		override CXXFLAGS := $(subst -std=c++11,-std=c++14,$(CXXFLAGS))
+		CXXFLAGS += -std=c++14
 
 		# The MSVC linker doesn't understand long section names
 		# generated by the clang compiler.