[AIX] unreachable code in basic_tools.m4 -> BASIC_CHECK_TAR

Thanks to Matthias for originally catching this issue.

basic_tools.m4 contains the line 'elif test "x$TAR_TYPE" = "aix";...'. This comparison can never be true, since no value of TAR_TYPE exists that will cause "x..." to equal "aix". The code following this line should be tested on AIX for hidden issues, then changed to the intended line: "x$TAR_TYPE" = "xaix";...' (by adding the missing 'x').