Add an osinclude.h below each OS directory, redirects by platform macro

* Linux folder has been renamed to Unix, to match defines and so that it
  compiles on OS X.
* This removes the need for a per-platform include search path for the
  right OS folder

CMakeLists.txt

@@ -7,8 +7,10 @@ project(glslang)
 if(WIN32)
     set(CMAKE_GENERATOR_TOOLSET "v110" CACHE STRING "Platform Toolset" FORCE)
     include(ChooseMSVCCRT.cmake)
+    add_definitions(-DGLSLANG_OSINCLUDE_WIN32)
 elseif(UNIX)
     add_definitions(-fPIC)
+    add_definitions(-DGLSLANG_OSINCLUDE_UNIX)
 else(WIN32)
     message("unkown platform")
 endif(WIN32)