Move SYSCONFDIR definition to makefile

Move SYSCONFDIR definition to makefile

Official autoconf docs contraindicate the use of the $sysconfdir
variable in configure.ac. For reference:

https://www.gnu.org/software/autoconf/manual/autoconf-2.63/html_node/Installation-Directory-Variables.html

The macro SYSCONFDIR (ordinarilly set to "/etc") is required to find
a system configuration file installed with i3. In some build setups,
this may not be defined properly in when the configure script is
compiled.

Instead, define this variable in AM_CPPFLAGS as the documentation
indicates.

fixes #2832