Fix cxx03 standard selection, option override in CMake 3.13+. Fixes #933 (#934)

CMakeLists.txt

@@ -6,6 +6,7 @@ foreach(p
     CMP0056 # export EXE_LINKER_FLAGS to try_run
     CMP0057 # Support no if() IN_LIST operator
     CMP0063 # Honor visibility properties for all targets
+    CMP0077 # Allow option() overrides in importing projects
     )
   if(POLICY ${p})
     cmake_policy(SET ${p} NEW)

test/CMakeLists.txt

@@ -148,7 +148,8 @@ if (BENCHMARK_HAS_CXX03_FLAG)
   compile_benchmark_test(cxx03_test)
   set_target_properties(cxx03_test
       PROPERTIES
-      COMPILE_FLAGS "-std=c++03")
+      CXX_STANDARD 98
+      CXX_STANDARD_REQUIRED YES)
   # libstdc++ provides different definitions within <map> between dialects. When
   # LTO is enabled and -Werror is specified GCC diagnoses this ODR violation
   # causing the test to fail to compile. To prevent this we explicitly disable