bug 858640: use platform-specific path separators so build scripts work on Windows

Revised patch that fixes bug 858640 by using platform-specific path separators so build scripts work on Windows when a build configuration includes an absolute LOCALES_FILE path.

Tested with the following commands on Windows:

    make
    LOCALES_FILE=locales/languages_dev.json make
    LOCALES_FILE='locales\languages_dev.json' make
    LOCALES_FILE='C:\Users\myk\Mozilla\gaia\locales\languages_dev.json' make

And these on Mac and Linux:

    make
    LOCALES_FILE=locales/languages_dev.json make
    LOCALES_FILE=$(PWD)/locales/languages_dev.json make