Recommit r363298 "[lit] Disable test on darwin when building shared libs."

Was reverted in r363379 due to build breakage.

Thanks to Nico Weber for reverting the original and suggesting the
fix.

Please see https://reviews.llvm.org/D61697

llvm-svn: 363502
diff --git a/clang/test/lit.cfg.py b/clang/test/lit.cfg.py
index c66ed9c..465b227d 100644
--- a/clang/test/lit.cfg.py
+++ b/clang/test/lit.cfg.py
@@ -183,3 +183,6 @@
 
 if os.path.exists('/etc/gentoo-release'):
     config.available_features.add('gentoo')
+
+if config.enable_shared:
+    config.available_features.add("enable_shared")