[CMake] Compiling 32- *and* 64-bit in same CMakeLists.txt file

[Chris Wood]

(Cross posted on Stackoverflow)

I'm trying to compile some code for 32 and 64 bit in the same
CMakeLists.txt file. I thought the easiest way to do it would be to
use a function. The (static) libraries used in the compilation are
also built in the CMakeLists.txt file. However, despite building them
in different directories, CMake complains that:

    add_library cannot create target "mylib" because another target with
    the same name already exists.  The existing target is a static library
    created in source directory "/home/chris/proj".

with the problem code being:

    cmake_minimum_required (VERSION 2.6 FATAL_ERROR)

    enable_language(Fortran)
    project(myproj)

    set(libfolder ${PROJECT_SOURCE_DIR}/lib/)

    function(build bit)

      message("Build library")
      set(BUILD_BINARY_DIR ${PROJECT_BINARY_DIR}/rel-${bit})
      set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${BUILD_BINARY_DIR}/bin)
      add_library(mylib STATIC ${libfolder}/mylib.for)
      set(CMAKE_Fortran_FLAGS "-m${bit}")

    endfunction()

    build(32)
    build(64)

I'm sure I'm missing something obvious, but can't see the problem...

Thanks,
Chris