docs/defines.txt: add a note about possibly obsolete __LDBL_COMPAT

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

docs/defines.txt

@@ -69,3 +69,5 @@ __UCLIBC_HAS_XXX__, __UCLIBC_HAVE_XXX__
 
 _IEEE_LIBM
     Always defined at libm build time
+__LDBL_COMPAT
+    Never defined, TODO: remove?