configure: update --enable-* help texts

All options in './configure --help' are indented in a way that the
description starts at the same position. So do it the same way to
improve readability.
The help texts are also more or less only repeating what the long
option implies. So write useful descriptions.
autotools: fix build with glib and multilib enabled

On 64 bit systems with the configure options --enable-multilib and
--enable-glib set we have the problem that the glib check only
detects if the 64 bit glib is available but not if a 32 bit glib
is available as well. So disable the 32 bit glib build in that
case. The result is that there are 32 bit libraries hooking the
standard C memory allocation functions and 64 bit libraries hooking
glib memory allocation functions like g_malloc() as well.