Fix parsing of Linux Makefile in configure.ac.

The variable MAKEARGS in the generated Linux kernel Makefile can contain
multiple arguments and not only the destination directory:

    MAKEARGS := -C /usr/src/linux-headers-3.10-3-common-rtai O=/usr/src/linux-headers-3.10-3-rtai-686-pae

Using sed to match only the destination directory (the part that comes after
"-C" and before "O=") fails in such cases because not only the path is printed
but also the remaining arguments:

        /usr/src/linux-headers-3.10-3-common-rtai O=/usr/src/linux-headers-3.10-3-rtai-686-pae

Signed-off-by: Shahbaz Youssefi <ShabbyX@gmail.com>