package/pkg-autotools.mk: fix runtime path

Commit 505b5a6c wrongly stated that "a
configure script bundled with a package may be generated with an older
version of Autotools, that may not yet support the --runstatedir option.
This will give an "Unsupported arguments" warning and the option will be
ignored.". Indeed, this will raise the following build failure with
attr, libtirpc, readline, etc.:

configure: error: unrecognized option: `--runstatedir=/run'

So to support configure scripts using older version, the variable is set
directly instead of using the --runstatedir argument.

Fixes:
 - http://autobuild.buildroot.org/results/c853f14773d875be30e32ae24daf80e181fef34e
 - http://autobuild.buildroot.org/results/045211cd66cf78c3d89d2d8ac41ec4edf0b0ee58
 - http://autobuild.buildroot.org/results/8c5160eef99de2f1f72f33d5e5ea94d417855b2c



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>