Add option to build with ASAN on OSS build

Summary:
Environment variable WITH_ASAN can be set to enable building
 with ASAN options.

 Ran into issues with getdeps.py extensions P137131160.
 Once that is resolved will do away with the ENV approach

 Requires building folly lib also with ASAN to avoid linking issues

Reviewed By: shri-khare

Differential Revision: D22860508

fbshipit-source-id: 48dea49378319972894f69b11bac6fa63ee20db9

CMake/folly-deps.cmake

@@ -194,6 +194,12 @@ option(
   "Build folly with Address Sanitizer enabled."
   OFF
 )
+
+if ($ENV{WITH_ASAN})
+  message(STATUS "ENV WITH_ASAN is set")
+  set (FOLLY_LIBRARY_SANITIZE_ADDRESS ON)
+endif()
+
 if (FOLLY_LIBRARY_SANITIZE_ADDRESS)
   if ("${CMAKE_CXX_COMPILER_ID}" MATCHES GNU)
     set(FOLLY_LIBRARY_SANITIZE_ADDRESS ON)