OTLog: re-enable exceptions on OSX

The _EXCEPTION_PTR_H macro was used to identify the incomplete
C++ exception implementation on some Android architectures and
hence skipping code that relies on it. Unfortunately, the
_EXCEPTION_PTR_H macro seems not to be defined in libc++ which
is used on OSX. Commit d01e411fdd27d9b66ebc6d70dedb8115fad31dfd
worked around the undefined macro by skipping the aforementioned
code which actually compiles on OSX just fine.
This commit restores exceptions on OSX by rearranging the macro
checks.
Merge pull request #374 from toxeus/fixOSXException

OTLog: re-enable exceptions on OSX