main: fix compiler warnings.
Add support for custom log formatting

This can be achieved by:
1. Subclass cpplog::LogMessage
2. Implement InitLogMessage
3. Call InitLogMessage from the constructor
4. Redefine the LOG_LEVEL macro

See examples/customlogformat.cpp
Replace deprecated strstream class.

This commit implements the fixed_streambuf class as a replacement for
strstream. fixed_streambuf is a safe implementation around
basic_streambuf and provides the needed functionality while preserving
the performance strstream provides.
Merge pull request #20 from olajep/pr_threadsafe_gmtime

Use thread-safe gmtime.
Merge pull request #17 from olajep/pr-compiler-warnings

main: fix compiler warnings.
Minor updates to Makefile / .gitignore
Merge pull request #19 from olajep/pr-replace-strstream

Replace deprecated class strstream.
Remove last usage of strstream
Merge pull request #18 from olajep/pr-custom-logformat

Add support for custom log formatting
Merge remote-tracking branch 'upstream/master'