mesa3d: fix build

We build mesa3d with dri support, which doesn't work with static builds,
so disable it.

From configure.ac:

dri)
    # DRI must be shared, I think
    if test "$enable_static" = yes; then
        AC_MSG_ERROR([Can't use static libraries for DRI drivers])
    fi

At the same time fixup minor style issues (:= vs =, long lines).

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>