configure.ac: Fix the order of some entries.

In order for lib autodetection and disabling of drivers (which require
libs that cannot be found) to work, the order of AC_ARG_ENABLEs,
AM_CONDITIONALs, and AC_DEFINEs needs to be changed.

All drivers start out enabled or disabled (depending on whether
the --disable-drivers option was supplied or not). Then, any driver can
be enabled or disabled explicitly via --enable-<drivername> or the
resp. --disable-<drivername> option. Finally, pkg-config checks for all
libraries are performed, and all drivers which require a library that
cannot be found are disabled explicitly (regardless of any
--enable-<drivername> option that might have been supplied).