cmake support set iOS Deployment Target for root project (#19315)

* support set ios deployment target for root project

* update gitignore

Author: drelaptop

.gitignore

@@ -179,3 +179,6 @@ tests/*/project/proj.android/.settings/org.eclipse.buildship.core.prefs
 # tmp folder for temp usage
 tmp/
 temp/
+
+# external libs zip
+*.zip

cmake/ios.toolchain.cmake

@@ -35,8 +35,9 @@ set(UNIX True)
 set(APPLE True)
 set(IOS True)
 
-# Required as of cmake 2.8.10
-set(CMAKE_OSX_DEPLOYMENT_TARGET "" CACHE STRING "Force unset of the deployment target for iOS" FORCE)
+# support iOS on cmake 3.11+
+# cmake 3.11 milestone feature, https://gitlab.kitware.com/cmake/cmake/issues/17431 
+set(CMAKE_OSX_DEPLOYMENT_TARGET "8.0" CACHE STRING "set of the deployment target for iOS" FORCE)
 
 # Determine the cmake host system version so we know where to find the iOS SDKs
 find_program(CMAKE_UNAME uname /bin /usr/bin /usr/local/bin)