gconf: Ensure that GCONF_SCHEMA_INSTALL_SOURCE uses correct paths
The gconf .m4 files use the path encoded into the gconf-native tools
by default to populate this variable. This doesn't work if we're
relocating the gconf-native binaries. By setting this variable we
ensure that the m4 files always pick up the correct path.
This fixes errors seen on the yocto autobuilder.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>