behaviour some versions of IRIX exhibit of committing suicide on big mallocs instead of just returning a friendly null pointer
when more than FD_SETSIZE file descriptors are open. This means that if for any reason we are not able to open more than FD_SETSIZE file descriptors then test 518 should not be run. test 537 is all about testing libcurl functionality when the system has nearly exhausted the number of free file descriptors. Test 537 will try to run with very few free file descriptors.
In this way we'll be able to sort out problems that might arise in the prechek phase of the 518 test. Once that 518 has been verified this change will be undone.
preventing execution of this test on many platforms