This fixes a "AX_COMPILER_VENDOR: command not found" error on
systems with older autotools versions (CentOS 5.x in this case).

Not a problem on newer auto* toolchains, they either ignore
acinclude.m4 completely or handle it in a different way.

(In fact, acinclude.m4 is not even needed for the one on CentOS 5,
 but we'll keep it for now.)

Tested-on: CentOS 5.7 x86_64 autoconf 2.59 / automake 1.9.6 / libtool 1.5.22
Tested-on: Gentoo 20111031 x86_64 autoconf 2.68 / automake 1.11 / libtool 2.4
Signed-off-by: Stefan Knoblich <stkn@openisdn.net>