[CMake] Workaround for PR24144: avoid installing libcxx_tsan and libcxx_msan by default "ninja install" command.

[samsonov]

Summary

Exclude external libc++ builds from "all" target, so that they are only
build on demand, and are not installed together with LLVM/Clang.

[hans]

Thanks! That seems like a good work-around.

compiler-rt/trunk/cmake/Modules/AddCompilerRT.cmake

foreach(flag ${LIBCXX_CFLAGS})
     set(flagstr "${flagstr} ${flag}")
   endforeach()
   set(LIBCXX_CFLAGS ${flagstr})
 
   if(NOT COMPILER_RT_STANDALONE_BUILD)
     list(APPEND LIBCXX_DEPS clang)
   endif()
 
+  set(EXCLUDE_FROM_ALL TRUE)
   ExternalProject_Add(${name}
     PREFIX ${prefix}
     SOURCE_DIR ${COMPILER_RT_LIBCXX_PATH}
     CMAKE_ARGS -DCMAKE_C_COMPILER=${COMPILER_RT_TEST_COMPILER}
                -DCMAKE_CXX_COMPILER=${COMPILER_RT_TEST_COMPILER}
                -DCMAKE_C_FLAGS=${LIBCXX_CFLAGS}
                -DCMAKE_CXX_FLAGS=${LIBCXX_CFLAGS}
                -DCMAKE_BUILD_TYPE=Release
macro(add_custom_libcxx name prefix)
 
   ExternalProject_Add_Step(${name} clobber
     COMMAND ${CMAKE_COMMAND} -E remove_directory <BINARY_DIR>
     COMMAND ${CMAKE_COMMAND} -E make_directory <BINARY_DIR>
     COMMENT "Clobberring ${name} build directory..."
     DEPENDERS configure
     DEPENDS ${LIBCXX_DEPS}
     )
+  set(EXCLUDE_FROM_ALL FALSE)
 endmacro()