SCons: Clean xatlas SCsub from thekla/poshlib defines

Those are not used in xatlas.

(cherry picked from commit eb155bc91abb4bdb293290fd4bc5ade12bc79669)

modules/xatlas_unwrap/SCsub

@@ -1,7 +1,5 @@
 #!/usr/bin/env python
 
-import platform
-
 Import('env')
 Import('env_modules')
 
@@ -21,23 +19,6 @@ if env['builtin_xatlas']:
     if (not env.msvc):
         env_xatlas_unwrap.Append(CXXFLAGS="-std=c++11")
 
-    if env["platform"] == 'x11':
-        # if not specifically one of the *BSD, then use LINUX as default
-        if platform.system() == "FreeBSD":
-            env_xatlas_unwrap.Append(CCFLAGS=["-DNV_OS_FREEBSD", "-DPOSH_COMPILER_GCC"])
-        elif platform.system() == "OpenBSD":
-            env_xatlas_unwrap.Append(CCFLAGS=["-DNV_OS_OPENBSD", "-DPOSH_COMPILER_GCC"])
-        else:
-            env_xatlas_unwrap.Append(CCFLAGS=["-DNV_OS_LINUX", "-DPOSH_COMPILER_GCC"])
-    elif env["platform"] == 'osx':
-        env_xatlas_unwrap.Append(CCFLAGS=["-DNV_OS_DARWIN", "-DPOSH_COMPILER_GCC"])
-    elif env["platform"] == 'windows':
-        if env.msvc:
-            env_xatlas_unwrap.Append(CCFLAGS=["-DNV_OS_WIN32", "-DNV_CC_MSVC", "-DPOSH_COMPILER_MSVC" ])
-        else:
-            env_xatlas_unwrap.Append(CCFLAGS=["-DNV_OS_MINGW", "-DNV_CC_GNUC", "-DPOSH_COMPILER_GCC", "-U__STRICT_ANSI__"])
-            env.Append(LIBS=["dbghelp"])
-
     env_thirdparty = env_xatlas_unwrap.Clone()
     env_thirdparty.disable_warnings()
     env_thirdparty.add_source_files(env.modules_sources, thirdparty_sources)