Fixes #146 (#211)

* Fixes #146

Author: leetal

ios.toolchain.cmake

@@ -156,7 +156,8 @@
 cmake_minimum_required(VERSION 3.8.0)
 
 # CMake invokes the toolchain file twice during the first build, but only once during subsequent rebuilds.
-if(DEFINED ENV{_IOS_TOOLCHAIN_HAS_RUN})
+# NOTE: To improve single-library build-times, provide the flag "OS_SINGLE_BUILD" as a build argument.
+if(DEFINED OS_SINGLE_BUILD AND DEFINED ENV{_IOS_TOOLCHAIN_HAS_RUN})
   return()
 endif()
 set(ENV{_IOS_TOOLCHAIN_HAS_RUN} true)