relax automake requirement to 1.15

The commit and discussion around it gave no reason why 1.16 is required.

All other repos have 1.8 to 1.11 as requirement.
In theory the version could be relaxed further.

Signed-off-by: Olaf Hering <olaf@aepfle.de>

configure.ac.in

@@ -55,7 +55,7 @@ AC_CANONICAL_TARGET()
 dnl ***************************
 dnl *** Initialize automake ***
 dnl ***************************
-AM_INIT_AUTOMAKE([1.16 dist-bzip2 tar-ustar no-dist-gzip foreign])
+AM_INIT_AUTOMAKE([1.15 dist-bzip2 tar-ustar no-dist-gzip foreign])
 AC_CONFIG_HEADERS([config.h])
 AM_MAINTAINER_MODE()
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])