[Log4cplus-devel] Fixes for CMake build files.

[Chernyshev V. <ast...@ro...>]

Hello.

After various unsuccessful attempts to compile log4cplus on OS X from 
provided autotools-generated configure file using Clang compiler I've 
decided to try CMake build. And it turned out that some configure 
defines are simply missing. Here are the fixes that I've made. All paths 
are relative to project's root directory.

1. ./CMakeLists.txt https://gist.github.com/3710553
    - added WITH_ICONV option that sets LOG4CPLUS_WITH_ICONV, the same
      is done when --with-iconv option is provided to configure system.
2. ./ConfigureChecks.cmake https://gist.github.com/3710633
    - added very simple check to find system-provided iconv library, it
      may be required to use iconv* functions on some systems;
    - added checks for functions mbstowcs, wcstombs, nanosleep;
    - added check for __func__ symbol;
    - iconv* functions will now be checked only when LOG4CPLUS_WITH_ICONV
      is defined, it mimics behaviour of autotools build system;
    - added proper checks for DECLSPEC_EXPORT/IMPORT/PRIVATE, taken from
      autotools files;
    - added thread-local support detection;
    - added C++11 atomics detection.
3. ./src/CMakeLists.txt https://gist.github.com/3710658
    - link iconv library if it is required to use iconv* functions and
      iconv support was requested.
4. ./include/log4cplus/config/defines.hxx.cmake.in
    https://gist.github.com/3710667
    - added missing defines to make it on par with defines.hxx.in.

There are additional things to tweak in CMake files, but it can be 
discussed later.