configure.ac: Remove obsolete(?) oddity

configure.ac: Remove obsolete(?) oddity

It causes build failure by adding the build system's includes to the
compiler's search path when cross-compiling with --prefix=/usr.

Building seems to work fine without it. It was added in f7274784. No idea
what it was trying to solve that couldn't be covered in a better way.

Change-Id: Ia32863f0b0cbd498eb34bd2fce73126db5b71a1f
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1530
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-by: Xiaofan <xiaofanc@gmail.com>