fix printing configure warning message when tex not installed
fix printing configure warning message when tex not installed

The value of $TEX is either 'tex' or 'false'. It is not empty if
tex is unavailable. The previous condition checked if $TEX was
a non-empty string (-n), and if so would print out a warning.
This was always the case, so a warning message would always print.

git-svn-id: ae38cf1093b87151f738d2b2220cac7742ad2e55@656 64e312b2-a51f-0410-8e61-82d0ca0eb02a

lubosz/check,lubosz/check,lubosz/check,lubosz/check