STYLE: don't use FIND_INCLUDE_FILE() but only FIND_INCLUDE_FILES() in
FindThreads.h

BUG: improve CheckC(XX)SourceRuns.cmake so that it works with cross
compiling, the return value has to go in the cache but shouldn't overwrite
the actual return value, and it should go only in the cache if we have a
result from try_run() otherwise we won't get here again if we have a result
later on

Alex