Tools: Fix configure with updated MinGW g++
I updated my MinGW / MSYS with mingw-get yesterday and today g++ crashes during configure for ScummVM Tools.
I traced the problem to configure's use of the environment's TMP variable. g++ 4.5.2 seems to expect this to be set to a valid temporary directory. It crashes the second time it is called because after the first call TMP is set to it's return value.
I simply changed configure to use RETVAL (what it's actually storing) instead of TMP in this patch.
Ticket imported from: #3309021. Ticket imported from: patches/1464.