Source grub-mkconfig_lib from the build directory at build time.

util/grub-install.in

@@ -34,6 +34,9 @@ platform=@platform@
 host_os=@host_os@
 pkglibdir="${libdir}/`echo ${PACKAGE_TARNAME}/${target_cpu}-${platform} | sed ${transform}`"
 datadir="@datadir@"
+if [ "x$pkgdatadir" = x ]; then
+    pkgdatadir="${datadir}/`echo @PACKAGE_TARNAME@ | sed "${transform}"`"
+fi
 localedir="@datadir@/locale"
 
 self="`basename $0`"
@@ -77,7 +80,7 @@ else
     disk_module=native
 fi
 
-. "${datadir}/@PACKAGE@/grub-mkconfig_lib"
+. "${pkgdatadir}/grub-mkconfig_lib"
 
 # Usage: usage
 # Print the usage.