Allow Indigo to compile under OS X 10.11

indigo-el-capitan.diff

diff --git a/common/cmake/GetSystemVersion.cmake b/common/cmake/GetSystemVersion.cmake
index 17615ab..794babd 100644
--- a/common/cmake/GetSystemVersion.cmake
+++ b/common/cmake/GetSystemVersion.cmake
@@ -27,6 +27,8 @@ elseif(APPLE)
             set(SUBSYSTEM_NAME "10.9")
         elseif(DARWIN_VERSION MATCHES 14)
             set(SUBSYSTEM_NAME "10.10")
+        elseif(DARWIN_VERSION MATCHES 15)
+            set(SUBSYSTEM_NAME "10.11")
         else()
             message(FATAL_ERROR "Unsupported DARWIN_VERSION: ${DARWIN_VERSION}")
         endif()