Test which CI runners use GTest >= 1.13

Makefile.inc

@@ -2,7 +2,7 @@
 CXX ?= g++
 CXXWARNINGS ?= -Wall -Wextra -Wundef -pedantic
 CXXINTRINSICS ?= -march=native
-CXXFEATURES ?= -funsafe-math-optimizations -fno-math-errno -std=c++14
+CXXFEATURES ?= -funsafe-math-optimizations -fno-math-errno -std=c++11
 CXXFLAGS ?= ${CXXWARNINGS} ${CXXINTRINSICS} ${CXXFEATURES} -g -O3
 
 # Delete this if OpenMP is not available (e.g., OS X without gcc)