[compiler-rt] Passthrough RUNTIMES_BUILD to the custom libc++ build

[phosek]

Summary

We use it to decide whether to include -nostdlib++ in the required
compiler flags. This addresses the issue introduced in rCRT354231
which broke the runtimes build.

[phosek]

D58333 is an alternative solution which is probably better than this one.

[phosek]

Abandoned in favor of D58333.

compiler-rt/cmake/Modules/AddCompilerRT.cmake

CMAKE_ARGS ${CMAKE_PASSTHROUGH_VARIABLES}
                -DCMAKE_C_FLAGS=${LIBCXX_C_FLAGS}
                -DCMAKE_CXX_FLAGS=${LIBCXX_CXX_FLAGS}
                -DCMAKE_BUILD_TYPE=Release
                -DLLVM_PATH=${LLVM_MAIN_SRC_DIR}
                -DLLVM_BINARY_DIR=${prefix}
                -DLLVM_LIBRARY_OUTPUT_INTDIR=${prefix}/lib
                -DCOMPILER_RT_LIBCXX_PATH=${COMPILER_RT_LIBCXX_PATH}
                -DCOMPILER_RT_LIBCXXABI_PATH=${COMPILER_RT_LIBCXXABI_PATH}
+               -DRUNTIMES_BUILD=${RUNTIMES_BUILD}
                ${LIBCXX_CMAKE_ARGS}
     INSTALL_COMMAND ""
     STEP_TARGETS configure build
     BUILD_ALWAYS 1
     USES_TERMINAL_CONFIGURE 1
     USES_TERMINAL_BUILD 1
     USES_TERMINAL_INSTALL 1
     EXCLUDE_FROM_ALL TRUE

compiler-rt/cmake/Modules/CustomLibcxx/CMakeLists.txt

 cmake_minimum_required(VERSION 3.4.3)
 
+# TODO: We shouldn't need a working C++ library, we're building C++ library
+# using this file after all. However, libc++abi and libc++ in standalone mode
+# include LLVM's HandleLLVMOptions which checks whether the compiler support
+# C++ using check_cxx_compiler_flag which tries to link C++ library and that
+# breaks C++ library hasn't been built yet (e.g. in the runtimes build where
+# C++ library is being built using the just built compiler).
+#
+# We need to prevent CMake from trying to use the C++ library. One way to do
+# that is by using -nostdlib and then only including -lc plus runtimes which
+# requires handling runtimes compiler-rt builtins and libgcc which adds another
+# layer of complexity. We could use -nostdlib++ but that flag is only supported
+# by newer Clang versions.
+#
+# For now, we pass the RUNTIMES_BUILD option through to detect whether we're
+# performing the runtimes build, and if so we use the -nostdlib++ flag.
+if (RUNTIMES_BUILD)
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdlib++")
+endif()
+
 # Build static libcxxabi.
 set(LIBCXXABI_STANDALONE_BUILD 1)
 set(LIBCXXABI_ENABLE_SHARED OFF CACHE BOOL "")
 set(LIBCXXABI_HERMETIC_STATIC_LIBRARY ON CACHE STRING "")
 set(LIBCXXABI_LIBCXX_PATH ${COMPILER_RT_LIBCXX_PATH} CACHE PATH "")
 set(LIBCXXABI_INCLUDE_TESTS OFF CACHE BOOL "")
 add_subdirectory(${COMPILER_RT_LIBCXXABI_PATH} ${CMAKE_CURRENT_BINARY_DIR}/cxxabi)