also set the minimum os/x version

with c++17, mac O.S. older than 10.13 do not support that

Signed-off-by: Kimball Thurston <kdt3rd@gmail.com>

cmake/OpenEXRSetup.cmake

@@ -19,6 +19,10 @@ if(CMAKE_CXX_STANDARD GREATER tmp)
 endif()
 set(OPENEXR_CXX_STANDARD "${tmp}" CACHE STRING "C++ standard to compile against")
 set(tmp)
+message(STATUS "Building against C++ Standard: ${OPENEXR_CXX_STANDARD}")
+# similar to the c++ standard, for mac os/x (and similar) targets, must
+# set a minimum version or newer standards (like 17) aren't available
+set(CMAKE_OSX_DEPLOYMENT_TARGET "10.13" CACHE STRING "Minimum Mac OS version")
 
 set(OPENEXR_NAMESPACE_CUSTOM "0" CACHE STRING "Whether the namespace has been customized (so external users know)")
 set(OPENEXR_INTERNAL_IMF_NAMESPACE "Imf_${OPENEXR_VERSION_API}" CACHE STRING "Real namespace for OpenEXR that will end up in compiled symbols")