Quoting from xorg-server/configure.ac:

if test "x$XF86VIDMODE" = xyes; then
    AC_MSG_NOTICE([Disabling XF86VidMode extension])
    XF86VIDMODE = no
fi

[ and then two more similar ones right after this one ]

Notice the spaces around the = sign when a value is given to a shell variable,
this is not valid in shell script. It leads to stupid messages such as
./configure: line 17692: XF86VIDMODE: command not found
and the extensions are actually not disabled, only the notice is printed.

-=-=-=-=-=-=-

I don't know if it is related or not, but later I get a linker error message
when compiling with "--enable-xwin":
[...]
i586-uhu-linux-gcc [tons of arguments]
../../dix/.libs/libdix.a(dispatch.o): In function `Dispatch':
dispatch.c:(.text+0x6f3): undefined reference to `ddxBeforeReset'
collect2: ld returned 1 exit status
make[4]: *** [Xorg] Error 1
make[4]: Leaving directory `...../hw/xfree86'
make[3]: *** [all-recursive] Error 1
[...]

Maybe I just simply shouldn't try to enable Xwin on Linux+gcc since it really
doesn't make much sense :-)