Fix issue with builddir when calling configure with absolute path

Calling configure with an absolute path fails on my machine:

    $ /home/j/repos/libffi/configure 
    checking build system type... x86_64-unknown-linux-gnu
    checking host system type... x86_64-unknown-linux-gnu
    checking target system type... x86_64-unknown-linux-gnu
    continue configure in default builddir "./x86_64-unknown-linux-gnu"
    ....exec /bin/bash ..//home/j/repos/libffi/configure "--srcdir=.." "--enable-builddir=x86_64-unknown-linux-gnu" "linux
    gnu"
    /bin/bash: ..//home/j/repos/libffi/configure: No such file or directory

This is due to missing escaping of square brackets in m4/ax_enable_builddir.m4, which defeats the absolute path detection. The proposed patch adds escaping of square brackets, as well as detection of absolute Windows paths (not sure if that is useful here but it shouldn't hurt).