Cygwin needs "-std=gnu++11" instead.

Fixes #150.

Change-Id: Id067989ee58bc35c3f59a55b65f54f6e216166c1
Reviewed-on: https://code-review.googlesource.com/14570
Reviewed-by: Paul Wankadia <junyer@google.com>

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index dd302a4..3e22472 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -29,6 +29,9 @@
   # CMake defaults to /W3, but some users like /W4 (or /Wall) and /WX,
   # so we disable various warnings that aren't particularly helpful.
   add_compile_options(/wd4100 /wd4201 /wd4456 /wd4457 /wd4702 /wd4815)
+elseif(CYGWIN OR MINGW)
+  # See https://stackoverflow.com/questions/38139631 for details.
+  add_compile_options(-std=gnu++11)
 elseif(CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang")
   add_compile_options(-std=c++11)
 endif()