bootstrap: Fix libuv target platform conditions on macOS
In commit 714ce72882 (bootstrap: Make libuv available during bootstrap,
2017-09-13, v3.10.0-rc1~106^2) we modified libuv to use POSIX interfaces
during bootstrap. However, some code still expects `TARGET_OS_IPHONE`
to be defined on Apple platforms to either `0` or `1`. In order to
offer this, include `<TargetConditionals.h>` on Apple platforms even
when bootstrapping.
diff --git a/Utilities/cmlibuv/include/uv/unix.h b/Utilities/cmlibuv/include/uv/unix.h
index 82778ff..f30562e 100644
--- a/Utilities/cmlibuv/include/uv/unix.h
+++ b/Utilities/cmlibuv/include/uv/unix.h
@@ -47,6 +47,9 @@
#ifdef CMAKE_BOOTSTRAP
# include "posix.h"
+# if defined(__APPLE__)
+# include <TargetConditionals.h>
+# endif
#elif defined(__linux__)
# include "linux.h"
#elif defined (__MVS__)
