Add install target for man pages
Rename files gfx to graphics & sys to system
Move platform detection preprocessing to platforms.hpp & rename defines

	* NOMLIB_SYSTEM_WINDOWS -> NOM_PLATFORM_WINDOWS
	* NOMLIB_SYSTEM_OSX -> NOM_PLATFORM_OSX
	* NOMLIB_SYSTEM_LINUX -> NOM_PLATFORM_LINUX
Introduce debug definition NOMLIB_RELEASE_WITH_ASSERT
Rename: NOMLIB_DEBUG, NOMLIB_DEBUG_TRACE and NOMLIB_RELEASE_WITH_ASSERT
Relocate NOM_DUMP_VAR outside of debugging ifdefs
Rename NOM_RELEASE_WITH_ASSERT to NOM_DEBUG_ASSERT
Introduce DEBUG, DEBUG_TRACE, DEBUG_ASSERT options

	* BUILD_EXAMPLES & BUILD_DOCS options off by default
	* Remove BUILD_RELEASE option (if not DEBUG, we assume release)
	* Rename NOMLIB_BUILD_EXAMPLES to BUILD_EXAMPLES