[libc++] Re-apply workaround for D63883

In reality, this workaround is for the fact that LIBCXX_CXX_ABI=libcxxabi
can't be specified on Linux, since libc++abi isn't shipped with the system.
Since the build bots explicitly specify LIBCXX_CXX_ABI=libcxxabi, they fail
unless we apply the workaround.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@373385 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -148,6 +148,16 @@ set(LIBCXX_CXX_ABI "default" CACHE STRING
 set(CXXABIS none default libcxxabi libcxxrt libstdc++ libsupc++ vcruntime)
 set_property(CACHE LIBCXX_CXX_ABI PROPERTY STRINGS ;${CXXABIS})
 
+# FIXME: This is a temporary hack to get the buildbots working while D63883 is in flight.
+# Without this all the bots fail while building libc++
+if (DEFINED ENV{USER})
+  if (("$ENV{USER}" STREQUAL "buildbot") OR (("$ENV{USER}" STREQUAL "llvmbb") OR ("$ENV{USER}" STREQUAL "buildslave")))
+    if (LIBCXX_CXX_ABI STREQUAL "libcxxabi" AND NOT DEFINED LIBCXX_CXX_ABI_INCLUDE_PATHS)
+      message(WARNING "OVERRIDING BUILDBOT CONFIG")
+      set(LIBCXX_CXX_ABI "default" CACHE STRING "FIXME" FORCE)
+    endif()
+  endif()
+endif()
 # Setup the default options if LIBCXX_CXX_ABI is not specified.
 if (LIBCXX_CXX_ABI STREQUAL "default")
   find_path(