[libc][posix] create 'posix' folder and move related files into it

components/libc/posix/pthreads/SConscript

@@ -0,0 +1,22 @@
+from building import *
+from utils import VersionCmp
+
+cwd        = GetCurrentDir()
+src        = Glob('*.c')
+CPPPATH    = [cwd]
+CPPDEFINES = []
+
+# only enable POSIX.1b-1993 Real-time extensions
+libc_ver   = GetDepend('LIBC_VERSION')
+try:
+    ver = libc_ver.split(' ')
+    ver = ver[1]
+    if VersionCmp(ver, "2.5.0") == 1:
+        CPPDEFINES = ['_POSIX_C_SOURCE=199309L']
+except :
+    pass
+
+group = DefineGroup('libc', src, 
+    depend = ['RT_USING_PTHREADS', 'RT_USING_LIBC'], CPPPATH = CPPPATH, CPPDEFINES = CPPDEFINES)
+
+Return('group')