CMakeLists: fix compilation flags

in order to build with cygwin, '-std=c++11' flag does not allow cygwin/mingw
compiler to use gnu (non-standard) functions, '-std=gnu++11' does

benchmark/CMakeLists.txt

 # Common variables.
 SET (CMAKE_BUILD_TYPE "Release")
-SET (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
+SET (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=gnu++11")
 SET (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Wmissing-include-dirs -Wundef -Wfloat-equal -Wshadow -Wunsafe-loop-optimizations")
 SET (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wdouble-promotion -Winit-self -Wvector-operation-performance -Wnoexcept -Weffc++ -Wstrict-null-sentinel")
 SET (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Woverloaded-virtual -Wsign-promo")

test/CMakeLists.txt

 # Common variables.
-SET (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
+SET (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=gnu++11")
 SET (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Wmissing-include-dirs -Wundef -Wfloat-equal -Wshadow -Wunsafe-loop-optimizations")
 SET (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wdouble-promotion -Winit-self -Wvector-operation-performance -Wnoexcept -Weffc++ -Wstrict-null-sentinel")
 SET (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Woverloaded-virtual -Wsign-promo")