Resync opam with mirage-dev

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
configure: allow the user to override the detected settings

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
configure: understand (but ignore) the standard configure arguments

These are the ones provided by (eg) CentOS 6's .spec %configure macro:

  ./configure --build=x86_64-redhat-linux-gnu --host=x86_64-redhat-linux-gnu
              --target=x86_64-redhat-linux-gnu --program-prefix= --prefix=/usr
              --exec-prefix=/usr --bindir=/usr/bin --sbindir=/usr/sbin
              --sysconfdir=/etc --datadir=/usr/share --includedir=/usr/include
              --libdir=/usr/lib64 --libexecdir=/usr/libexec
              --localstatedir=/var --sharedstatedir=/var/lib
              --mandir=/usr/share/man --infodir=/usr/share/info

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #55 from djs55/configure

Improve the configure script