Fix compiler warnings.

This commit is based on pull request #1493
fix compiler warnings
winpr/comm: don't build on apple

Since code is only defined on linux building comm causes /usr/bin/ranlib
to warn about empty objects and the test doesn't build at all on apple.
Fix warnings found in Xcode
fix comment style

// to /* */
Merge pull request #1985 from bmiklautz/fix/warnings

Fix compiler warnings and build