Support Meson build system

Support for Meson build system has been added thanks to @nazar-pc

Support correct shared library creation

To build code with shared version of wingetopt, include

#define WINGETOPT_SHARED_LIB

or add -DWINGETOPT_SHARED_LIB to command line options.
In 99.9% cases using static wingetopt is better (nothing needs to be defined)