[libc++abi] Make sure we can run the tests in Standalone mode

The tests would previously fail if the `python` executable wasn't found,
because we were missing the mandatory find_package.

libcxx/utils/ci/run-buildbot.sh

@@ -201,7 +201,8 @@ legacy-standalone)
           -DCMAKE_INSTALL_PREFIX="${INSTALL_DIR}" \
           -DLLVM_PATH="${MONOREPO_ROOT}/llvm" \
           -DLIBCXXABI_LIBCXX_PATH="${MONOREPO_ROOT}/libcxx" \
-          -DLIBCXXABI_LIBCXX_INCLUDES="${MONOREPO_ROOT}/libcxx/include"
+          -DLIBCXXABI_LIBCXX_INCLUDES="${MONOREPO_ROOT}/libcxx/include" \
+          -DLIBCXXABI_LIBCXX_LIBRARY_PATH="${BUILD_DIR}/libcxx/lib"
 
     echo "+++ Building libc++abi"
     ninja -C "${BUILD_DIR}/libcxxabi" cxxabi

libcxxabi/CMakeLists.txt

@@ -48,6 +48,23 @@ if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR OR LIBCXXABI_STANDALONE_B
   include(HandleOutOfTreeLLVM)
 endif()
 
+if (LIBCXXABI_STANDALONE_BUILD)
+  find_package(Python3 COMPONENTS Interpreter)
+  if(NOT Python3_Interpreter_FOUND)
+    message(WARNING "Python3 not found, using python2 as a fallback")
+    find_package(Python2 COMPONENTS Interpreter REQUIRED)
+    if(Python2_VERSION VERSION_LESS 2.7)
+      message(SEND_ERROR "Python 2.7 or newer is required")
+    endif()
+
+    # Treat python2 as python3
+    add_executable(Python3::Interpreter IMPORTED)
+    set_target_properties(Python3::Interpreter PROPERTIES
+      IMPORTED_LOCATION ${Python2_EXECUTABLE})
+    set(Python3_EXECUTABLE ${Python2_EXECUTABLE})
+  endif()
+endif()
+
 # Require out of source build.
 include(MacroEnsureOutOfSourceBuild)
 MACRO_ENSURE_OUT_OF_SOURCE_BUILD(