Fixes strange C++11 compatibility problems during GTest installation:

 /tmp/gtest-20200216-20679-mu1044/googletest/include/gtest/internal/gtest-internal.h:1249:1: error: unknown type name 'constexpr'
 constexpr bool TypedTestCaseIsDeprecated() { return true; }

 /tmp/gtest-20200216-20679-mu1044/googletest/include/gtest/internal/gtest-internal.h:1249:11: error: expected unqualified-id
 constexpr bool TypedTestCaseIsDeprecated() { return true; }

 /tmp/gtest-20200216-20679-mu1044/googletest/include/gtest/internal/gtest-internal.h:1254:1: error: unknown type name 'constexpr'
 constexpr bool RegisterTypedTestCase_P_IsDeprecated() { return true; }