Fixing -rpath specification for Darwin.

Reported by Doctor Rockit.
Only create log file when installing.

This small patch avoids accumulating unnecessary log files due invalid
command line invocations by moving the log file creation to a later point in
time.
Merge pull request #3 from mavam/master

Avoid accumulating excess log files
Merge branch 'llvm33'
Merge remote-tracking branch 'origin'