build-sys: improved check for fallocate()

build-sys: improved check for fallocate()

With glibc 2.10 on a 32bits system, fallocate64() function is not
exported. This a problem, since _FILE_OFFSET_BITS is set to 64 and
fallocate() is redirected to fallocate64().
Sadly, AC_CHECK_FUNC() doesn't catch such problem, since it's overriding
the function prototype.

See this for references:
http://sources.redhat.com/ml/libc-hacker/2009-05/msg00003.html

Signed-off-by: Yann Droneaud <yann@droneaud.fr>