Added 'macros' keyword to build_api.py to allow building USB libraries

This morning I received an error message when running `python workspace_tools/build.py -u -m KL25Z -t GCC_ARM` stating that `build_lib()` did not expect the `macros` keyword argument. I noticed that `build_lib()` just calls `build_library()` which _does_ accept the `macros` keyword, so I just changed `build_lib` to pass it through.

I built the mbed SDK from a clone of the repo this morning on a Windows XP machine using Python 2.7.5.  This change appears to have allowed a successful build of the USB libraries on my machine, but I haven't used mbed to program any hardware yet, nor have I tested it on any other systems.